THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 

GIFT  OF 

H.  L.  Masser 


MECHANICAL 


DRAFT 


A   PRACTICAL  TREATISE 


B.    F.    STURTEVANT    CO. 

BOSTON,   MASS. 

NEW  YORK.  PHILADELPHIA. 


STURTEVANT  ENGINEERING  CO. 

LONDON.  GLASGOW. 

STOCKHOLM.  BERLIN.  MILAN.  AMSTERDAM. 


CATALOGUE  No. 


COPYRIGHT,   1898,  BY  B.  F.   STURTEVANT  CO. 


PRESS  OF  CARL   H.    HEINTZEMANN,   BOSTON,  MASS. 


•ssr 

?v5 


Introduction. 


The  subject  of  mechanical  draft  has  been  discussed  at  greater  or  less  length  in  the 
technical  press  and  before  various  engineering  societies ;  but  in  all  cases  such  discus- 
sion has  been  distinctly  limited.  Here,  for  the  first  time,  the  attempt  is  made  to  give 
the  treatment  that  its  importance  demands.  Although  its  introduction  is  evidence  of  a 
somewhat  radical  departure  in  certain  features  of  boiler  practice,  yet  extended  and 
recent  experience  clearly  indicates  the  permanence  of  this  departure.  Hereafter,  the 
progressive  engineer  will  not  adopt  the  chimney  as  the  sole  means  of  draft  produc- 
tion, but  will  first  carefully  consider  the  respective  advantages  of  the  chimney  and 
the  fan,  for  the  claims  of  the  latter  to  superiority  cannot  be  disregarded. 

Manifestly,  the  individual  who  in  the  present  day  is  to  decide  between  the  two 
methods,  requires  a  clear  presentation  of  their  respective  merits,  such  as  is  attempted 
in  this  work.  Its  object  is  two-fold  :  First,  to  instruct  by  a  lucid  discussion  of  the 
entire  subject,  with  such  supplementary  information  as  may  be  necessary  to  show 
the  superiority  of  mechanical  draft.  Second,  to  show  the  special  adaptability  of  the 
Sturtevant  fans  for  this  purpose,  and  to  indicate  in  some  degree  the  extent  to  which 
they  have  already  been  applied.. 

With  the  desire  to  give  the  reader  all  the  information  that  may  be  required  for  a 
full  understanding  of  the  subject,  without  necessitating  reference  to  other  works, 
the  chapters  on  Water,  Steam  and  Fuels  have  been  incorporated.  In  the  chapters 
on  Efficiency  of  Fuels,  and  of  Boilers,  the  principles  which  pertain  to  the  efficiency, 
convenience  and  adaptability  of  mechanical  draft  are  presented  in  their  abstract 
relation  to  the  subject  in  general.  In  so  far  as  such  statements  are  general  in  their 
character  their  treatment  is  that  of  a  text-book.  But  wherever  they  particularly 
concern  the  utility  of  mechanical  draft  and  the  employment  of  fans  for  its  produc- 
tion, they  are  substantiated  by  quotations  from  eminent  authorities,  and  in  all  cases 
references  are  introduced.  An  impartial  character  is  thus  given  to  all  statements, 
while  the  actual  facts  are  doubly  emphasized. 


The  last  two  chapters  are  of  especial  value  and  interest,  inasmuch  as  they  forcibly 
indicate  the  methods  of  application  under  a  great  variety  of  conditions,  and  are 
witnesses  to  the  possibilities  of  this  method  of  draft  production.  In  so  far  as 
possible,  all  statements  regarding  the  application  and  operation  of  Sturtevant  fans, 
as  well  as  those  relating  to  the  character  and  advantages  of  certain  other  systems 
or  devices,  have  been  quoted  and  the  references  given.  This  fact  should  be  carefully 
noted,  for  the  purpose  has  been  to  entirely  eliminate  the  element  of  personal  prej- 
udice and  thereby  avoid  even  the  appearance  of  either  favorable  or  adverse  comment. 
To  the  reader  who  can  give  but  little  time  to  this  discussion,  Chapter  XI  serves  as 
a  summary  of  the  advantages  of  this  system. 

Every  endeavor  has  been  made  to  render  this  work  authoritative,  to  discuss  the 
subject  in  all  its  aspects  and  to  give  assurance  of  the  most  impartial  treatment.  In 
the  light  of  the  foregoing  it  is  presented  in  confidence  that  its  careful  perusal  will 
result  in  conviction  as  to  the  superiority  of  mechanical  draft. 

B.  F.  STURTEVANT  CO. 


Table  of  Contents, 


CHAPTER  I. 

WATER.  Page. 

Composition   .         .         .         .         .         .         .         .         ,         .         .         .         .  ,         i 

Weight  and  Bulk ....  i 

Expansion  by  Heat .     "    .  .  .      2 

Specific  Heat      .         .         ....;......  3 

Unit  of  Heat ' 4 

Mechanical  Equivalent  of  Heat          .         .         .         .         ...         .         .         ,  5 

Pressure  of  Water         .         .         .         ,         .         .         ....         .  .         5 

Impurities  in  Water    .         .         .         .         .         .         .                            .         .         .  6 

CHAPTER  II. 

STEAM. 

Composition            .         .         .         .         ......         .         .         .         .  .         7 

Weight  and  Bulk .         4         .-..-".  7 

Expansion  by  Heat  —  Absolute  Zero      .         .         .         .         .    •    .       ..         .  . ...      f 

Specific  Heat '.         .         .         .         ..       .         -.  12 

Latent  and  Sensible  Heat      .         .         .         .                   .                   ...         •  •       12 

Flow  of  Steam            .         .         .         .         .         ."         .      -  .      .  ^ ..  -    *         .         .  13 

Steam-Pipe  Coverings    .         .         .         .         .        _.       ~  .         .         .         ...        14 

CHAPTER  III. 

COMBUSTION7. 

Definition        .          .          .          .          .          .          .          .          .          .          .  .16 

Carbon 16 

Oxygen  .         .         .         . ' 1 6 

The  Atomic  Theory   ............  17 

Union  of  Carbon  and  Oxygen         .         .         .         .         ,         .         .         .         .  .19 

Combustion  of  Fuel    ......'......  20 

Air  Required  for  Combustion          .         .         .         .                   .         .         .         .  .22 

Air  for  Dilution 25 

Analysis  of  Flue  Gases  ...........       28 

Heat  of  Combustion           ...........  30 

Ideal  Temperature  of  Combustion          .         .         .         .         .         .         .         .  .32 


vi  TABLE    OF  CONTENTS. 

CHAPTER  IV. 

FUELS.  Page. 

Definition -3$ 

Natural  Fuels     ....;.  36 

Artificial  Fuels       .         .         .         .         .         .      .    .'       .  36 

Wood .         .         .         .         .  37 

Straw  and  Tan       . .  38 

Bagasse      .         .         .  •        .      * 39 

Peat       ......  42 

Coal            ........  44 

Lignite           .         .         .....         .         .                  .         .         .         .  -45 

Bituminous  Coal         ........  46 

Semi-Bituminous  Coal    .         .         ...         .         .         •         .         .  48 

Semi-Anthracite  Coal          .         .......         .         •'.'•'••         •         •  48 

Anthracite  Coal      .         .                  ._.,-.         .         .         .         .         .         .         .  .48 

Geographical  Classification         .         .    '     .         .         .         .                   .         .         .  49 

Petroleum      .         .         .         ...         .         .         '.         .         ....  53 

Natural  Gas       .         .         .         . .  .53 

Artificial  Fuels       .                            .         .         .         ...         .               '  .         .  54 

Charcoal     .......         ;•        .         .      '  .         .         .         .  55 

Coke      .......  -55 

Fuel  Gas            ...,../  56 

Patent  Fuel   .         .         .         .         .         .         ....         .         .         .         .  .       57 

CHAPTER  V. 

EFFICIENCY  OF  FUELS. 

Measure  of  Efficiency    .         .         .                  ...         .         .         .         .         .  •       59 

Relative  Efficiency  of  Various  Coals     •                        .         .         .         .         ...  61 

Influence  of  Ash    .         .         .         .         .         .         .         .         .                   .         .  .66 

Influence  of  Moisture  in  Coal    .         .         .         .         .         .         .         ...  67 

Influence  of  Size  of  Coal      .         .         .         *         .         .         .         .         ...       67 

Influence  of  Air  Supply     ....         .         .         .         '.         .         .  72 

Influence  of  the  Frequency  of  Firing    .         .'       .         .         .         .         .         .  -73 

Loss  on  Account  of  Smoke         .         .         .     •    .         .         .         .         .         .         .  74 

Loss  on  Account  of  Carbonic  Oxide      .         .         .•        .         .     ' 75 

Admission  of  Air  above  the  Fire .         .  78 

Loss  on  Account  of  Excess  of  Air .  .80 

Summary  of  Influences  Affecting  the  Efficiency  of  Fuel    .....  85 

Commercial  Efficiency  of  Coals     .         .         .         .         .         .         .         .         .  .87 

Influence  of  Mechanical  Draft            .         .         .         .         .         .         .         .         .  91 

Prevention  of  Smoke     ............       92 


TABLE    OF  CONTENTS.  vii 
CHAPTER  VI. 

EFFICIENCY    OF    STEAM    BOILERS.  Page. 

Measure  of  Efficiency    ............       95 

Rating  of  Steam  Boilers    ...........  98 

Radiation  and  Convection  of  Furnace  Heat 102 

Distribution  of  the  Heat  of  Combustion    .         .         .         .         .         .         .         .  104 

Disposition  of  Heat  in  Steam  Boilers    .         .         .         .         .         .         .  .106 

Sources  of  Efficiency          .         .         .         .         .         .         .         .         ...         ,  107 

Flue  Feed-Water  Heaters  or  Economizers      .         .         .         .         .         .  .112 

Air  Heaters  or  Abstractors .  n6 

Increased  Tube-Heating  Efficiency         .          .         .         .         .         .         .         .  .118 

Mechanical  Stokers    .         .         .         .         .         .         .         .         .         ...         .  121 

Powdered-Fuel  Furnaces        .         .         .         .         .         .         .         .         .         .  .123 

Influence  of  Mechanical  Draft 

on  the  Ultimate  Efficiency  of  Steam  Boilers    .          .         .         *.         .         .  123 

CHAPTER  VII. 

RATE  OF  COMBUSTION. 

Rate  of  Combustion       .         .         .         .         .         .                   .         .         .         .  .130 

Relation  of  Grate  Surface  to  Heating  Surface  .         .         .         .         .         .         .  131 

Economy  of  High  Rates  of  Combustion         . 132 

Thickness  of  Fire       .         .         .         .         .         .         .                   .         .         .         .  140 

CHAPTER  VIII. 

DRAFT. 

Definition       .         .         .         .         ......         .         .         .         .  .      141 

Relation  of  Pressure  and  Velocity      .         .                   .         .         .         .         .         .  141 

Efflux  of  Air          .         .         .         .         .         .         .         ....         .         .  .      144 

Influence  of  Form  of  Orifice      .         .         .         .         .....         .         .  151 

Work  Required  to  Move  Air           .         .         .         .      .-.'_'.         .     :•  .         .  -152 

Movement  of  Air  in  Pipes          .         .         .         .         .         .         .         .         .         .  155 

Measurement  of  Draft  .         .         .         .         .         .                   .         .         .         .  .159 

Conditions  of  Boiler  Draft           ..........  165 

Relation  of  Draft  and  Rate  of  Combustion   .         .         .         .         .         ...  .169 

Leakage  of  Air  ............177 

CHAPTER  IX. 

CHIMNEY  DRAFT. 

Principles  of  Chimney  Draft           .         .         .         .         .         .         .         .         .  .     1 79 

Chimney  Design 181 

Efficiency  of  Chimney    .....  ......     191 


viii  TABLE    OF  CONTENTS. 

CHAPTER    X. 

MECHANICAL   DRAFT.  Page. 

Definition       .         .         .."-•.         ... 196 

Steam  Jets          .         ....      .         .         .         •         •         •         •  ;"'•'        •                   •  19& 

Fans      .         .         .         .         .         :         .         .         .         ..     ;    .         .  .198 

Theory  of  Fans          .         .         .         .         .         .         .'        ..         .         .         .         .  200 

Design  of  Fans     .         ....         .  " "     .         .         .         .         .         .         .         .  .     204 

Methods  of  Application     .         .         .         ..        .   •      ,         .         «         .                   .  213 

Closed  Ashpit  System     .         .         .         ,         .         .    '   -.         .        -v         .         .  .     214 

Closed  Fire-room  System    .         .         .         .         .         .         .  .       .         .         .         .  217 

Induced  System      .         .         .         .         .         .         •         •         .         • -.  .    ••'•''„•  •     22° 

CHAPTER    XI. 

ADVANTAGES  OF  MECHANICAL  DRAFT. 

General  Advantages       .         .         .         .    -  . .         .         .         .       '  .         .         .  .     223 

Necessity   .          .                   .         .        '.          .          .          .          .          .          .          .          .  228 

Adaptability           .         . 228 

Controllability .                  .         .  229 

Flexibility      .         .         .•       .         . '   .  .     230 

Independence  of  Climatic  Conditions          .         .         .         .         .         .         ...  230 

Portability      .         .         .         .                  .         .         ...         .         .                  .  .     231 

Salability ...         .         ...  .232 

Efficiency  of  Fan  vs.  Chimney        .                  .         .         .         .                            .  .     232 

Omission  of  Chimney          .         .         .         .         .         .         .         .                  .         .  233 

Increased  Rate  of  Combustion       ....                                     ...  233 

Efficiency  of  Combustion  .         .         .         .         .                  '.                            .         .  233 

Burning  Cheap  Fuels     .         .         .         .                  .    •               .         .                   .  .     234 

Economy  in  Quantity  of  Fuel     .          .          .         .          .          .         .         ...          .  235 

Mechanical  Stokers .         .         .    '     ,         .  .     235 

Smoke  Prevention       .         .         .         .         .         .         »         .         .                  .         .  236 

Utilization  of  Waste  Heat  in  Gases .         .  .     236 

Economy  in  Space      .          .          .          .         .          .                   .         .         ....  237 

Economy  in  First  Cost  .          .          .          .         .                  "...          .          .  .      237 

Decreased  Size  of  Boiler  Plant  for  Given  Output       .         .         .         .         .         .  238 

Economy  in  Operating  Expenses    .          .          .          .          .          t         .         .  .     240 

Ventilation          .         .         .         :         ..         .         .       .  .         ......         .  240 

Summary  of  Advantages        .          .          .          ...          .          .          .          .  .     241 

CHAPTER    XII. 

THE    STURTEVANT    FANS    FOR    MECHANICAL    DRAFT. 

Steel  Pressure  Blower    ............  244 

"  Monogram  "  Blower          ...........  245 


TABLE    OF  CONTENTS.  ix 

Page. 

"  Monogram  ''  Blower  on  Adjustable  Bed        ........  246 

ii  Monogram  "  Blower  on  Adjustable  Bed  with  Engine         .....  247 

Steel-Plate  Blower 249 

Steel-Plate  Blower  on  Adjustable  Bed  with  Engine     ......  250 

Steel-Plate  Exhauster     .         .         .         .         .         .         .  .         .  -251 

Steel-Plate  Steam  Fan        ...........  252 

Steel-Plate  Steam  Fan  with  Engine  Enclosed         .         .         .         .         .  -       .         .  254 

Steel-Plate  Exhauster  with  Inlet  Connection       .         .         .         .         ...  255 

Special  Steel-Plate  Steam  Fans      ........          ."'''••  256 

Double  Upright  Enclosed  Engine        .........  260 

Special  Steel-Plate  Steam  Fan,  Double  Enclosed  Engine,  Cylinders  beneath  the 

Shaft 261 

Special  Duplex  Steel-Plate  Steam  Fan 263 

Special  Steel-Plate  Steam  Fan  with  Upright  Compound  Engine     ....  263 

Special  Steel  Plate  Steam  Fan  with  Double  Open-Type  Engine           ...  264 

Special  Cast-Iron  Steam  Fan  with  Double  Horizontal  Engine        .         .         .          .  265 

Steel-Plate  Steam  Fan  with  Three-Quarter  Housing  ......  267 

Steel-Plate  Steam  Fan  with  Three-Quarter  Housing  and  Double  Upright  En- 
gine        .         .         .         .         .         ....         .         .         .         .         .  268 

Steel-Plate  Steam  Fan,  Three-Quarter-Housing  Type,  with  Steel-Plate  Bottom    .  269 

Electric  Fan,  ••  Monogram  "  Pattern      .         .         .         .         .         .         .         .         .  271 

Electric  Fan,  Steel-Plate  Pattern        ... 272 

CHAPTER    XIII. 

APPLICATION    OF    THE    STURTEVANT   FANS    FOR    MECHANICAL   DRAFT. 

Typical  Arrangement  of  the  Sturtevant  Steam   Fan    for    the    Production   of 

Under-Grate  Forced  Draft      . 275 

Ashpit  Dampers          .                            .........  275 

Crystal  Water  Company,  Buffalo,  N.  Y.          .         .         .         .         .         .         .         .  277 

B.  F.  Sturtevant  Co.,  Jamaica  Plain,  Mass.         .         .         .         .         .         .         .  279 

U.  S.  Navy 291 

U.  S.  S.  Swatara 291 

American  Line  Pier  14,  N.  R.,  International  Navigation  Company,  New  York, 

N.  Y 293 

Steamer  L.  C.  Waldo .297 

Gordon  Hollow  Blast  Grate 299 

Gadey  Air  Grate         .                                     301 

Cheney  Brothers,  South  Manchester,  Conn.    ........  303 

The  Deringer  Colliery  of  the  Cross  Creek  Coal  Company,  Deringer,  Pa.    .         .  305 

Central  Unidad,  Cuba 310 

L.  B.  Darling  Fertilizer  Company,  Pawtucket,  R.  I.    .         .  313 

Cleveland  Iron  Mining  Company,  Ishpeming,  Mich.        .         .         .         .         .         .  314 


x  TABLE    OF  CONTENTS. 

Page 

John  Brown  &  Company,  Limited,  Sheffield.  England     .         .         .         .         .  .317 

Union  Traction  Company,  Philadelphia,  Pa.       . 320 

Dust  Destructors  at  Shoreditch,  London,  England 324 

United  States  Cotton  Company,  Central  Falls,  R.I..         .         .         .         .         .  327 

Glens  Falls  Paper  Mill  Company,  Fort  Edward,  N.  Y.            .         .         .         .  .331 

Crane  &  Breed  Manufacturing  Company,  Cincinnati,  Ohio          ....  333 

Holyoke  Street  Railway  Company,  Holyoke,  Mass.         .         .         .         .         .  -335 

Fair  Alpaca  Company,  Holyoke,  Mass.            •    .                            •         •         •         •  337 

The  Pope  Tube  Company,  Hartford,  Conn.    .         .         .         .         .         .         .  .     341 

S.  S.  St.  Louis  and  St.  Paul        .      '  .         ...         .         .                   .         .         .  343 

S.  S.  Kensington  and  Southwark    ..........     348 

Washington  Garbage  Crematory,  Washington,  D.  C.           .'*        .     .    .         .         .  353 

Knickerbocker  Lime  Company,  Mill  Lane  Station,  Pa.            .         .         .         .  .     354 

Boston  Woven  Hose  and  Rubber  Company,  Cambridgeport,  Mass.    .         .         .  356 

Crematory,  Lee  and  Pennsylvania  Avenues,  Washington.  D.  C.                .         .  .     358 

The  Riordon  Paper  Mills,  Limited,  Merriton.  Ont.      .         .         .         .         .       - .  360 

Boston  Duck  Company,  Bondsville,  Mass.      .         „         ...         .  •'..         .  .361 

Hotel  Iroquois,  Buffalo,  N.  Y.     .         ...         .         .         .         .         .         .  364 

C.  Kiener  Fils,  Eloyes,  France       .                  .         .         .                  ...  .     366 

"  Incognito "  Plantation,  La.        .         .         .         .                  . .        .         .•         .         .  366 

Guaranty  Building  Company,  Buffalo,  N.  Y.          ..:        .  "-     .         .         .         .  .     367 

Stamford  Gas  and  Electric  Company,  Stamford,  Conn.      .         .         .        . .         .  369 

Societe  Alsacienne  de  Constructions  mecaniques    .         .         ...         .  .     370 

Leon  Godchaux.  Elm  Hall  Plantation.  La.           .         .         .         .         ..         .         .  371 

Otto  Colliery,  Philadelphia  &  Reading  Coal  and  Iron  Co.,  Branchdale,  Pa.    .  .372 


CHAPTER  I. 
WATER. 

Composition.  —  Pure  water,  whether  solid,  liquid  or  gaseous,  is  a  chemical 
combination  of  the  two  elements  hydrogen  and  oxygen  in  the  unalterable  pro- 
portion of  two  parts,  by  volume,  of  the  former  to  one  part  of  the  latter.  A 
mere  mechanical  mixture  of  hydrogen  and  oxygen  remains  still  a  mixture  until, 
through  the  influence  of  heat,  electricity  or  other  special  agents,  the  two  chem- 
ically combine.  Such  a  result  may  be  brought  about  by  introducing  a  lighted 
taper  in  a  properly  proportioned  mixture  of  the  gases,  the  resulting  water  being 
deposited  as  dew  upon  the  sides  of  the  containing  vessel.  If  the  union  of  the 
hydrogen  and  oxygen  be  effected  in  an  apparatus  so  arranged  that  the  water 
formed  by  the  combination  is  kept  at  such  a  high  temperature  that  it  remains 
in  the  gaseous  condition,  it  will  be  found  that  the  two  volumes  of  hydrogen  and 
one  volume  of  oxygen  which  were  mixed  together  have  become  compacted  into 
two  volumes  of  steam,  as  the  result  of  the  chemical  union. 

Conversely,  two  volumes  of  water  in  its  vaporized  or  gaseous  form  may,  by 
various  methods,  be  decomposed  into  its  constituent  elements;  namely,  two 
volumes  of  hydrogen  and  one  volume  of  oxygen.  Principal  among  the  methods 
of  dissociation  is  the  application  of  heat.  Consequently  the  presence  of  mois- 
ture in  fuel  may  assume  considerable  importance  in  the  ordinary  process  of 
combustion  in  connection  with  steam  boilers. 

Weight  and  Bulk.  —  Water*  is  universally  adopted  as  the  standard  by  which 
the  relative  weights  of  all  liquids  and  solids  are  determined,  this  relation  being 
expressed  by  the  term  "  specific  gravity."  The  specific  gravity  of  a  body, 
therefore,  indicates  its  weight  as  compared  with  that  of  an  equal  volume  of 
pure  water.  Determinations  of  specific  gravity  are  generally  referred  to  the 
weight  of  one  cubic  foot  of  water  at  62°  F.  At  the  more  important  tempera- 
tures the  weights  are  as  follows :  — 

Weight  of  One  Cubic  Foot  of  Pure    Water. 

At  32°  F.        (freezing  point)         '    .     .     .     .     62.418  pounds. 

"  39.1°  F.     (maximum  density) 62.425         " 

"  62°  F.        (standard  temperature) 62.355        " 

"  212°  F.     (boiling  point  under  atmospheric  pressure)      .     .      59.640         " 


MECHANICAL   DRAFT. 


For  general  purposes  the  weight  of  water  is  taken  in  round  numbers  as  62.5 
pounds  per  cubic  foot.  The  calculated  weights  at  temperatures  from  32°  to 
290°  are  given  in  Table  No.  i. 

In  bulk,  water  is  usually  measured  by  the  gallon,  the  volume  of  which  is  231 
cubic  inches  (the  British  gallon  contains  277.274  cubic  inches),  or  0.134  cubic 
feet.  A  gallon  of  water  at  62°,  therefore,  weighs  slightly  over  8^/3  pounds,  and 
7.48  gallons  equal  one  cubic  foot. 

Table  No.  i.  — Expansion  and  Density  of  Pure  'Water. 


Temper- 
ature. 
Degrees 
Fahr. 

Absolute 
Pressure 
of  Vapor 
per  gq.  in. 
Pounds. 

Relative 
Volume. 
Water  at 
32°=  i. 

Relative 
Density. 
Water  at 
32°='. 

Density  or 
Weight 
of  i  Cubic 
Foot. 
Pounds. 

Temper- 
ature. 
Degrees 
Fahr. 

Absolute 
Pressure 
of  Vapor 
per  sq.  in. 
Pounds. 

Relative 
Volume. 
Water  at 
32°=  i. 

Relative 
Density. 
Water  at 
32°=  i. 

Density  or 
Weight 
of  i  Cubic 
Foot. 
Pounds. 

320 

0.089 

I.OOOOO 

1  .00000 

62.418 

J350 

2.542 

I-OI539 

0.98484 

61.472 

35 

O.I  00 

Q-99993 

1  .00007 

62.422 

140 

2.879 

1.01690 

0.98339 

61.381 

40 

0.122 

0.99989 

I.OOOII 

62.425 

MS 

3-273 

1.01839 

0.98194 

61.291 

45 

0.147 

0.99993 

1  .00007 

62.422 

I50 

3.708 

1.01989 

0.98050 

61.201 

5° 

0.178 

1.00015 

0.99985 

62.409 

I5S 

4-193 

1.02164 

0.97882 

61.096 

55 

0.214 

i  .00038 

0.99961 

62.394 

1  60 

4-731 

1.02340 

0.97714 

60.991 

60 

0.254 

1.00074 

0.99926 

62.372 

I65 

5-327 

1.02589 

0.97477 

60.843 

65 

0.304 

1.00119 

0.99881 

62.344 

170 

5-985 

1  .02690 

0.97380 

60.783 

70 

0.360 

1.  00  1  60 

0.99832 

62.313 

*7S 

6.708 

1.02906 

0.97193 

60.665 

75 

0.427 

1.00239 

0.99771 

62.275 

1  80 

7-5" 

1.03100 

0.97006 

60.548 

80 

0.503 

1  .00299 

0.99702 

62.232 

185 

8-375 

1.03300 

0.96828 

60.430 

85 

0.592 

1.00379 

0.99622 

62.182 

190 

9-335 

1.03500 

0.96632 

60.314 

90 

0.693 

1.00459 

Q-99543 

62.133 

'95 

10.385 

1.03700 

0.96440 

60.198 

95 

0.809 

1.00554 

0.99449 

62.074 

200 

11.526 

1.03889 

0.96256 

60.081 

IOO 

0.940 

1  .00639 

0-99365 

62.O22 

20S 

12.770 

1.0414 

0.9602 

59-93 

105 

1.095 

1.00739 

0.99260 

61.960 

210 

F4.I26 

1.0434 

0.9584 

59.82 

no 

1.267 

1.00889 

0.99119 

61.868 

212 

14.7 

1.0444 

0-9575 

59-76 

"5 

1.462 

1.00989 

0.9902  1 

6l.8o7 

230 

20.87 

1.0529 

0.9499 

59-36 

120 

1.685 

1.01139 

0.98874 

61.715 

250 

29.80 

1.0628 

0.9411 

58.75 

I25 

1.932 

1.01239 

0.98808 

61.654 

270 

41.87 

1.0727 

0.9323 

58.18 

130 

2.215 

1.01390 

0.98630 

61.563 

290 

57-64 

I.o838 

0.9227 

57-59 

Expansion  by  Heat.  —  Although  water  is  practically  non-compressible,  even 
under  the  most  extraordinary  pressure,  it  readily  expands  by  the  mere  applica- 
tion of  heat,  with  the  notable  exception  that  between  the  temperature  of  melting 
ice  at  32°  and  that  of  greatest  density  at  39.1°  there  is  a  gradual  contraction  in 
volume  as  heat  is  applied. 


MECHANICAL   DRAFT. 


The  rate  of  expansion  is,  however,  variable,  and  above  212°  but  little  is 
known  regarding  it  experimentally.  An  application  of  the  formula  derived 
from  experiments  made  below  212°  gives  at  least  approximate  values.  In 
Table  No.  i  are  embodied  the  results  of  calculation  by  Rankine's  approximate 
formula,  which  gives  substantially  correct  values  at  low  and  moderate  tempera- 
tures. The  results  at  high  temperatures  are,  however,  too  large,  as  is  evidenced 
by  the  fact  that  by  the  table  the  density  at  212°  is  shown  to  be  59.76  pounds 
per  cubic  foot,  while  by  actual  measurement  it  has  been  found  to  be  59.64 
pounds, —  an  error,  however,  of  only  0.002  in  the  value,  which  is  of  small  ac- 
count except  in  refined  calculations. 

Specific  Heat.  —  Bodies  vary  greatly  in  the  capacity  which  they  possess  for 
absorbing  heat  under  equal  changes  in  temperature.  The  relation  which  thus 
exists  between  them  is  expressed  by  the  "  specific  heat, "  which  may  be  defined 
as  the  quantity  of  heat  necessary  to  be  imparted  to  a  given  body  in  order  to 
raise  its  temperature  one  degree  relatively  to  the  quantity  that  is  required  to  raise 
through  one  degree  an  equal  weight  of  water  from  its  point  of  greatest  density 
at  39.1°.  Thus,  for  instance,  one  pound  of  air  at  constant  pressure  maybe 
raised  through  one  degree  by  the  expenditure  of  only  0.2375  °f  trie  heat  nec- 
essary to  raise  one  pound  of  water  through  one  degree ;  or,  what  amounts  to  the 
same,  the  amount  of  heat  expended  to  raise  the  temperature  of  one  pound  of 


water  by  one  degree  would  heat 


4.2105  pounds    of   air    through   the 


0-2375 

same  increment.  As  the  specific  heat  of  water  is  greater  than  that  of  any  other 
known  substance,  the  specific  heat  of  all  other  substances  must  of  necessity  be 
expressed  in  decimals.  Water  does  not  absorb  heat  exactly  in  proportion  to  its 
increase  in  temperature ;  in  other  words,  the  specific  heat  of  water  varies  with 
the  temperature,  as  is  rendered  evident  by  Table  No.  2. 

Table  No.  2.  —  Specific  Heat  of  Water. 


Temperature. 
Degrees 
Fahrenheit. 

Specific  Heat 
at  the 
Given  Temperature. 
Freezing  point  =  i. 

Temperature. 
Degrees 
Fahrenheit. 

Specific  Heat 
at  the 
Given  Temperature. 
Freezing  point  =  i. 

Temperature. 
Degrees 
Fahrenheit. 

Specific  Heat 
at  the 
Given  Temperature. 
Freezing  point  =  i. 

3~ 

1  .0000 

176° 

.0089 

3200 

1.0294 

5° 

1.0005 

194 

.0109 

338 

1.0328 

68 

1.  001  2 

212 

.0130 

356 

1.0364 

86 

1.0020 

230 

•0153 

374 

I.O4OI 

104 

I  .OOJO 

248 

.0177 

394 

1.0440 

122 

1.0042 

266 

I.O2O4 

410 

1.0481 

140 

I.O056 

284 

1.0232 

428 

1.0524 

I58 

1.0072 

302 

1.0262 

446 

1.0568 

MECHANICAL    DRAFT. 


Unit  of  Heat. — The  quantitative  measure  of  heat  is  the  thermal  unit.  The 
British  thermal  unit  (as  distinguished  from  the  French  thermal  unit,  or  calorie) 
is  that  quantity  of  heat  which  is  required  to  raise  the  temperature  of  one  pound 
of  pure  water  through  one  degree  Fahr.  at  or  near  39.1°  Fahr.,  the  temperature 
of  maximum  density  of  water.  As  employed  in  general  practice,  the  term  is 
usually  abbreviated  to  "  B.  T.  U." 

The  relation  existing  between  the  temperature  of  water  in  degrees  Fahrenheit 
and  the  number  of  thermal  units  contained  therein,  together  with  the  increase  in 
the  number  of  thermal  units  for  each  increment  of  temperature  of  5  degrees,  is 
indicated  in  Table  No.  3. 

Table  No.  3.  —  Number  of  Thermal  Units  Contained  in  One  Pound  of  Water. 


Temper- 
ature. 
Degs.  F. 

Number  of 
Thermal 
Units. 

Increase. 

Temper- 
ature. 
Degs.  F. 

Number  of 
Thermal 
Units. 

Increase. 

Temper- 
ature. 
Degs.  F. 

Number  of 
Thermal 
Units. 

Increase. 

35° 

35-000 

155° 

155-339 

5-034 

2750 

276.985 

5.107 

40 

40.001              5-OOI 

1  60 

160.374 

5-035 

280 

282.095 

5-IIO 

45 

45-002 

5-001 

I65 

165.413 

5-039 

285 

287.210 

5-"5 

5° 

50.003 

5-OOI 

170 

170.453 

5-040 

290 

292.329 

5-"9 

55           55-oo6 

5-003 

'75 

175-497 

5.044 

295 

297.452 

5-123 

60 

60.009 

5.003 

180 

180.542 

5-045 

300 

302.580 

5.128 

65 

65.014 

5.005 

185 

185.591 

4.049 

305 

307.712 

5-132 

70 

70.020 

5.006 

190 

190.643 

5.052 

3IO 

312.848 

5-'36 

75 

75.027 

5.007   ;     195 

195.697 

5.054 

315 

3I7.988 

5.140 

80           80.036 

5.009        |         200 

200.753 

5.056 

320 

323-I34 

5.146 

85 

85-045 

5.009 

205 

205.813 

5.060 

325 

328.284 

5-  '50 

90 

90.055 

5.010 

2IO 

210.874 

5.061 

330 

333-438 

5-154 

95 

.  95-067 

5.012 

2IS 

215-939 

5-065 

335 

338-596 

5-I58 

100 

100.080 

5-013 

220 

22I.OO7 

5.068 

340 

343-759 

5-163 

105 

105.095 

5-01  5 

225 

226.078 

5-071 

345 

348.927 

5.168 

no 

IIO.IIO 

5-015 

230 

23i-'53 

5-075 

35° 

354.101 

5-'74 

"5 

115.129 

5.019 

235 

236.232 

5.079 

355 

359.280 

5-J79 

120 

120.149 

5.020 

240 

24I-3U 

5.081 

360 

364.464 

5.184 

'25 

125.169 

5.0:10 

245 

246.398 

5-085 

365 

369-653 

5.189 

I30 

130.192 

5.023 

250 

251.487 

5.089 

370 

374.846 

5-193 

135 

i35-2T7 

5-025 

255 

256-579 

5.092 

375 

380.044 

5.198 

140 

140.245 

5.028 

200 

261.674 

5-095 

380 

385-247 

3.203 

'45 

I45-I75 

5.030 

265 

266.774 

5.100 

385 

390.456 

5.209 

150 

1  50-305 

5.030       270 

271.878 

5.104 

390 

395.672 

5.216 

MECHANICAL   DRAFT.  5 

Mechanical  Equivalent  of  Heat.  —  The  mechanical  unit  of  work  is  the  "foot- 
pound," or  the  work  required  to  raise  one  pound  through  a  distance  of  one 
foot.  The  mechanical  theory  of  heat  regards  heat  as  a  mode  of  motion,  and 
investigation  has  shown  that  there  exists  a  definite  relation  between  these  two 
forms  of  energy,  which  is  known  as  the  "mechanical  equivalent  of  heat."  That 
is,  if,  as  in  the  experiments  of  Joule,  a  certain  known  amount  of  mechanical 
energy  is  expended  (as  by  the  falling  of  a  weight)  to  operate  paddles  in  a 
vessel  of  water,  the  increase  in  temperature  of  the  water,  due  to  agitation  by 
the  paddles,  will  always  be  found  to  be  proportional  to  the  work  done.  This 
relation  or  proportion  is  universally  expressed  by  the  amount  of  work  necessary 
to  raise  the  temperature  of  one  pound  of  water  through  one  degree  Fahrenheit, 
and  the  latest  experimental  determinations  of  Rowland  show  it  to  be  practically 
778  foot-pounds. 

Pressure  of  Water.  —  From  the  weight  of  water  at  the  standard  temperature 
of  62°,  its  pressure  upon  any  exposed  surface  may  be  readily  determined  for  any 
given  depth  or  head.  The  weight  of  one  cubic  foot  at  the  above  temperature 
being  62.355  pounds,  it  is  evident  that  for  a  head  of  one  foot  the  pressure  must  be 

62.355  pounds  per  square  foot,  and  — ^^  =  0.433  pounds  per  square  inch  ;  and, 
further,  that  a  pressure  of  one  pound  per  square  inch  will  be  produced  by  a 
head  of  — =  2.309  feet.  Upon  this  basis  Table  No.  4  has  been  calculated 
to  show  the  head  corresponding  to  different  pressures. 


Table  No.  4.  — Head  in  Feet  of  Water 
Corresponding  to  Pressures  in  Pounds  per  Square  Inch. 


Pres- 
sure. 

0 

• 

- 

3 

4 

5 

6 

7 

8 

9 

0 

2.309 

4.619 

6.928 

9.238 

"•547 

I3-857 

16.166 

18.476 

20.785 

10 

23-°95 

25.404 

27.714 

30.023 

32-333 

34.642 

36-952 

39.261 

4I-570 

43-880 

20 

46.189 

48.499   50-808 

53.118 

55427 

57-737 

60.046 

62.356 

64.665 

66.975 

3° 

69.284 

71-594 

73-903 

76.213 

78.522 

80.831 

83.141 

85.450 

87.760 

90.069 

40 

92.379 

94.688 

96.998 

99-307 

101.62 

103-93 

106.24 

108.55 

110.85 

113-16. 

5° 

"5-474 

117.78 

120.09 

122.40 

I247I 

1  26.02 

129-33 

131.64 

133-95 

136.26 

60 

138.568 

140.88 

I43-I9 

145-5° 

I47.8I 

150.12 

152.42 

154-73 

I57-04 

159-35 

70 

161.663 

163-97 

166.28 

168.59 

170.90 

173.21 

J75-52 

I77.83 

180.14 

182.45 

80 

184.758 

187.07 

189.38 

191.69 

194.00 

196.31 

198.61 

200.92 

203.23 

205.54 

90 

207.852 

210.16 

212.47 

214.78 

217.09 

219.40 

221.71 

224.02 

226.33 

228.64 

MECHANICAL   DRAFT. 


Impurities  in  Water.  —  Water,  as  present  in  nature,  is  always  more  or  less 
contaminated  by  impurities,  so  that  for  all  refined  measurements  in  which  water 
plays  the  part  of  a  standard  it  must  be  in  its  distilled  state ;  that  is,  absolutely 
pure. 

In  boiler  practice,  the  exact  composition  of  the  water  is  of  marked  import- 
ance, as  upon  the  impurities  held  in  suspension  or  solution  may  depend,  not 
only  the  economy  of  operation,  but  the  very  life  of  the  boiler  itself.  The  more 
common  impurities,  the  character  of  the  trouble  which  they  cause  and  the 
remedy  or  palliation  to  be  applied,  are  indicated  in  the  following  list :  — 

Remedy  or  Palliation. 
(  Filtration. 
(_  Blowing  off. 
Blowing  off. 

(  Heating  feed,  addition  of  cau- 
\  stic  soda,  lime  or  magnesia,  etc. 
(  Addition  of  carbonate  of  soda, 
\  barium  chloride. 
(  Addition  of  carbonate  of  soda, 
(etc. 

„  .    .  (  Addition    of    barium    chloride, 

Priming.  J 

(etc. 

Corrosion. 


Troublesome  Substance. 
Sediment,  mud,  clay,  etc. 

Readily  soluble  salts. 
Bicarbonate  of  lime,  magnesia 
and  iron. 

Sulphate  of  lime. 


Chloride  or  sulphate  of  magnesium. 

Carbonate  of  soda  in  large 

amounts. 

Acid  (in  mine  waters). 
Dissolved  carbonic  acid  and 

oxygen. 


Trouble. 
Incrustation. 
Incrustation. 
Incrustation. 

Incrustation. 
Corrosion. 


Corrosion. 


Grease  (from  condensed  steam).          Corrosion. 

Organic  matter  (sewage).  Priming. 

Organic  matter.  Corrosion. 


Alkali. 

(  Heating  feed,  addition  of  cau- 
\  stic  soda,  slaked  lime,  etc. 
f  Slaked  lime  and  filtering,  car- 
J  bonate  of  soda,  substitute  min- 
(  eral  oil. 

(  Precipitate  with  alum  or  ferric 
(  chloride  and  filter. 

Precipitate  with  alum  or  ferric 

chloride  and  filter. 


CHAPTER   II. 
STEAM. 

Composition.  —  Water  in  its  aeriform  condition  is  to  be  considered  as  a  vapor 
rather  than  as  a  gas,  the  term  "gas"  applying  more  properly  to  a  body  which, 
under  ordinary  temperature  and  pressure,  continually  remains  in  its  aeriform 
state.  In  its  vaporous  condition,  water  exists  in  the  atmosphere  in  various  pro- 
portions and  under  different  conditions  of  atmospheric  pressure,  as  indicated  in 
Table  No.  5.  The  slow  process  by  which  this  production  of  aqueous  vapor 
takes  place  at  the  free  surface  of  a  liquid  is  generally  termed  evaporation,  while 
the  more  rapid  production  of  the  vapor  throughout  the  mass  is  commonly  desig- 
nated as  boiling.  Under  either 'method  of  vaporization  the  change  is  merely 
physical,  the  constituent  elements  remaining  the  same  in  character  and  propor- 
tion ;  namely,  two  parts  of  hydrogen  and  one  part  of  oxygen,  by  volume. 

Weight  and  Bulk.  —  When,  by  the  application  of  heat,  aqueous  vapor  is  pro- 
duced from,  and  in  contact  with,  water  in  a  closed  vessel,  it  is  usually  denomi- 
nated "steam,"  and  under  these  conditions  is  always  saturated.  Saturated 
steam  is  of  varying  density  and  temperature,  according  to  the  pressure  under 
which  it  is  generated ;  but  there  exists  an  unalterable  relation  between  density, 
elastic  force  and  temperature,  such  that  if  one  of  these  properties  remains  con- 
stant the  others  so  remain,  while  a  change  in  one  results  in  a  change  of  the 
other  two,  and  always  in  a  fixed  ratio. 

The  elastic  force  or  pressure  at  low  temperatures  is  shown  in  column  4  of 
Table  No.  5,  while  that  at  higher  temperatures,  as  expressed  in  pounds  per  square 
inch  above  vacuum,  is  presented  in  Table  No.  6.  In  columns  5  and  6  of  this 
table  are  also  given  the  weights  and  volumes  of  steam  under  the  corresponding 
temperatures  and  pressures.  The  specific  density  of  gaseous  steam  is  0.622, 
that  of  air  being  i .  That  is  to  say,  the  weight  of  a  cubic  foot  of  gaseous  steam  is 
about  five-eighths  of  that  of  a  cubic  foot  of  air  of  same  pressure  and  temperature. 

Expansion  by  Heat,  Absolute  Zero.  —  When  saturated  steam  is  superheated 
or  surcharged  with  heat,  as  may  only  be  done  when  it  is  separated  from  the 
water  from  which  it  was  generated,  it  advances  from  the  condition  of  saturation 
to  that  of  gaseity.  The  gaseous  state  is  only  arrived  at  by  considerably  eleva- 
ting the  temperature,  supposing  the  pressure  remains  the  same.  Obviously,  the 


8  MECHANICAL   DRAFT. 

test  of  perfect  gaseity  must  be  the  uniformity  of  the  rate  of  expansion  with  the 
rise  in  temperature.  Experiment  has  shown  that,  with  the  exception  of  a  slight 
variation  at  a  temperature  just  above  that  at  which  it  was  generated, .  steam, 
thus  superheated,  follows  the  law  that  controls  the  expansion  of  permanent 
gases :  this  law  being  that,  the  temperature  remaining  the  same,  the  volume  of 
a  given  quantity  of  gas  is  inversely  proportional  to  the  pressure  which  it  sus- 
tains ;  or,  conversely,  that,  the  pressure  remaining  the  same,  the  volume  will 
be  proportional  to  the  temperature.  Therefore,  the  density  or  volume  at  any 
given  temperature  being  known,  it  may  be  readily  determined  by  proportion  for 
any  other  temperature. 

This  rate  or  coefficient  of  expansion  of  a  perfect  gas  per  degree,  as  deter- 
mined by  the  most  recent  and  refined  experiments,  is  0.00203  = at  the 

492.66 

freezing  point,  or  32°  F.  Or,  in  other  words,  for  each  rise  in  temperature  of  one 
degree,  the  gas  of  freezing  temperature  increases  in  volume -?-? ,  and  an  in- 
crease of  492.66°  would  double  the  volume.  If,  then,  this  law  holds  good, 
reckoning  upward  from  freezing  point,  as  has  been  conclusively  proven  by 
experiment,  it  is  reasonable  to  suppose  that  it  likewise  holds  good  reckoning 
downward,  and  that  for  every  degree  of  temperature  withdrawn  from  the  gas  it 

is  diminished  — — 77   of  its   volume  at  32°.      Carried   to  its   limit,   this  would 
492.00 

point  to  the  fact  that  at  492.66°  below  freezing  (or  460.66°  below  zero  Fahren- 
heit) the  contraction  would  be  equal  to  the  volume ;  that  is,  the  volume  would 
cease  to  exist.  This  is  the  so-called  absolute  zero  of  temperature,  the  starting- 
point  for  the  scale  of  absolute  temperatures  by  which  the  proportional  expan- 
sion of  gases  is  determined.  Without  appreciable  error  it  may  be  expressed  in 
round  numbers  as  461°  below  zero  Fahrenheit,  and  will  be  so  understood  in 
calculations  which  follow. 

The  action  of  steam  under  the  imposed  conditions  of  constant  pressure, 
.  superheating  and  separation  from  water  is  not  to  be  confounded  with  that  which 
takes  places  in  the  ordinary  boiler  or  in  the  cylinder  of  a  steam  engine.  In  the 
case  of  a  boiler,  an  increase  in  the  temperature  of  the  steam  cannot  occur  with- 
out an  equal  increase  in  that  of  the  water,  which  results  in  the  generation  of 
more  steam  and  an  increase  in  pressure  ;  in  fact,  the  relations  are  such  that  the 
temperature  of  the  steam  may  always  be  determined  from  a  knowledge  of  the 
pressure,  and  vice  versa. 

In  an  engine  cylinder,  on  the  other  hand,  the  steam  in  the  process  of  expan- 
sion expends  a  certain  portion  of  its  energy  in  work,  its  pressure  and  temperature 


MECHANICAL   DRAFT. 


are  both  reduced,  and  its  volume  is  increased.  But,  owing  to  condensation 
by  loss  of  heat  transformed  into  work  and  to  local  conditions,  the  rate  of  expan- 
sion under  the  decreasing  pressure  departs  from  the  law  of  a  perfect  gas. 

Table  No.  5.  —  Weights  of  Air,  Vapor  of   Water,  and  Saturated  Mixtures    of  Air   and 

Vapor  at  different  Temperatures,  under  the  ordinary  Atmospheric 

Pressure  of  29.921  Inches  of  Mercury. 


j  Volume  of 
Tern-  i  Dry  Air  at 
pera-     different 
ture.  1  Tempera- 
Degs.  tures,    the 
Fahr-  Volume  at 
enheit    32°  being 
!      i.ooo. 

Weight  of 
a  Cubic 
Foot  of 
Dry  Air  at 
different 
Tempera- 
Pounds. 
3 

Elastic 
Force  of 
Vapor  in 
Inches  of 
Mercury, 
Regnault. 

4 

Mixtures  of  Air  saturated  with  Vapor. 

Cubic  Ft. 
of  Vapor 

of    Water 
at  its  own 
Pressure 

Column  4 

Elastic 
Force  of 
the  Air  in 
the  Mix- 
ture of  Air 
and  Vapor 
in  Ins.  of 
Mercury. 
5 

Weight  of  Cubic  Foot  of  the 
Mixture  of  Air  and  Vapor. 

Weight  of 
Vapor 
mixed 
with  i  Ib. 
of  Air,  in 
Pounds. 

9 

Weight  of 
Dry  Air 
mixed 
with  i  Ib. 
of  Vapor, 
n  Pounds 

10 

Weight  of 
he  Air,  in 
Pounds. 

6 

Weight  of 
the  Vapor, 
n  Pounds 

7 

Total 
Weight  of 
Mixture, 
in  Pounds 

8 

°°       -935 

.0864 

.044 

29.877 

.0863 

.OOOO79 

.086379 

.00092 

1092.4 

12 

.960 

.0842 

.074 

29.849 

.0840 

.000130 

.084130 

•COI55 

646.1 

22 

.980 

.0824 

.118 

29.803 

.082! 

.O00202 

.082302 

.00245 

406.4 

32          I  -OOO 

.0807 

.181 

29.740 

.0802 

.000304 

.080504 

.00379 

263.81 

3.289 

42       i.oco 

.0791 

.267 

29.654  ' 

.0784 

.000440 

.078840 

.00561 

178.18 

2,252 

S2 

I.04I 

.0776 

.388 

29-533 

.0766 

.000627 

.077227 

.00819 

122.17 

'.595 

62 

1.061 

.0761 

•556 

29-365 

.0747 

.000881 

.075581 

.01179 

84.79 

IU35 

72 

1.082 

.0747 

.785 

29.136 

.0727 

.OOI  22  I 

.073921 

.01680 

59-54 

819 

82 

1.  102 

•0/33 

1.092 

28.829 

.0706 

.001667 

.072267 

.02361 

42-35 

600 

92 

1.  122 

.0720 

1.501 

28.420 

.0684 

.002250 

.070717 

.03289 

30.40 

444 

102 

I-M3 

.0707 

2.036 

27.885 

.0659 

.002997 

.068897 

•04547 

21.98 

334 

112 

I.l63 

.0694 

2./3I 

27.190 

.0631 

.003946 

.067046 

.06253 

T5-99 

253 

122 

I.I84 

.0682 

3.621 

26.300 

•0599 

.005142 

.065042 

.08584 

11.65 

194 

132 

1.204 

.0671 

4-752 

25.169 

.0564 

.006639 

.063039 

.11771 

8.49 

'51 

I42 

1.224 

.0660 

6.165 

23-756 

.0524 

.008473 

.060873 

.16170 

6.18 

nS 

152 

1.245 

.0649 

7-93° 

21.991 

.0477 

.OIO7I6 

.058416 

.22465 

4-45 

93-3 

162 

1.265 

.0638 

10.099 

19.822 

.0423 

.013415 

•055715 

•3I7I3 

3-15 

74-5 

I72 

1.285 

.0628 

12.758 

17.163 

.0360 

.016682 

.052682 

.46338 

2.16 

59-2 

182 

1.306 

.0618 

15.960 

13.961 

.0288 

.020536 

•049336 

.71300 

1.402 

48.6 

192 

1.326 

.0609 

19.828 

10.093 

.0205 

•025142 

.045642 

1.22643 

.815 

39-8 

202 

1-347 

.0600 

24.450 

5-471 

.0109 

.030545 

.041445 

2.80230 

•357 

32-7 

212 

1-367 

.0591 

29.921 

o.ooo 

.OOOO 

.036820 

.036820 

Infinite. 

.000 

27.1 

In  fact,  a  steam  engine  is  a  device  for  transforming  heat  into  work.  In  so  far  as 
all  other  losses  are  prevented  it  becomes  a  perfect  heat  engine,  performing  solely 
the  function  of  transformation.  The  indicator  diagram  serves  to  show  the  actual 
process  of  expansion  as  it  takes  place  in  the  cylinder,  subject  to  practical 
conditions. 


MECHANICAL   DRAFT. 


Table  No.  6.  —  Properties  of  Saturated  Steam. 


Total  Pres- 
sure, in  Ibs. 
per  sq.  in., 
measured 

Vacuum. 

Temperature,  in 
Degrees  Fahren- 
heit of  Steam 
and  of  the  Water 
from  which  it 
was  evaporated. 

2 

Number  of  British  Thermal  Units 
contained  in  one  Pound,  reckoned 
from  Zero  Fahrenheit. 

Weight  of 
one  Cubic  Foot 
of  Steam,  in 
Decimals  of  a 
Pound. 

5 

Volume  of 
one  Pound  of 
Steam,  in  Cubic 
Feet. 

6 

Relative  Volume 
or  Cubic  Feet 
of  Steam  from 
one   Cubic  Foot 
of  Water. 

7 

Numberrequired 
For  Evaporation, 
known  as  Latent 
Heat,  or  Heat  of 
Vaporization. 
3 

Total  Number 
contained 
in  the  Steam. 

4 

, 

102. 

1,042.96 

1,145.05 

.0030 

330.36          ,       2O,62O. 

2 

126.27 

I,O26.0I 

1,152.45 

.0058 

172.08          |       IO,720. 

3 

141.62 

1,015.25 

I.I57-I3 

.0085 

117.52                   7.326. 

4 

1  53-07 

1,007.23 

I,l6o.62 

.OI  I  2 

89.62                   5,600. 

5 

162.33 

1,000.73 

1,163.45 

•0137 

72.66              4,535- 

6 

I7O.I2 

995-25 

1,165.83 

.0163 

61.21              3,814. 

7 

176.91 

990.47 

1,167.90 

.0189 

52.94              3,300. 

8 

182.91 

986.25 

1,169.73 

.0214 

46.69              2,910. 

9 

188.32 

982.43 

I.I7L37 

.0239 

41.79 

2,607. 

10 

I93.24 

978.96 

1,172.88 

.0264 

37.84 

2,360. 

12 

201.96 

972.80 

1,175-54 

•0313 

3J-95 

1,988. 

14 

209.56 

967-43 

1,177.85 

.0362 

27.63 

1,722. 

14.7 

212. 

965.7 

1,178.60 

.0380 

26.36 

1,644. 

16 

216.30 

962.66 

1,179.91 

.0413 

24.21 

1,514. 

18 

222.38 

958-34 

1,181.76 

.0462 

21.64 

1,350.6 

20 

227.92 

954.41 

1,183.45 

.O5II 

19-57 

I,22O.3 

22 

233.02 

950-79 

1,185.01 

.0561 

17-83 

i,  "3-5 

24 

237-75 

947.42 

I,l86.45 

•  O6lO 

16.39 

1,024.1 

26 

242.17 

944.28 

1,187.80 

.0658 

i5-!9 

948.4 

28 

246.33 

941.32 

1,189.07 

.0707 

14.14 

883.2 

3° 

250.24 

938.92 

1,190.26 

•0755 

13-25 

826.8 

32 

253-95 

935.88 

I,I9I.39 

.0803 

12.45 

777-2 

34 

257.48 

933-37 

1,19247 

.0851 

"•75 

733-5 

36 

200.83 

930-97 

1,193-49 

.0899 

ii.  ii 

694-5 

38 

264.05 

928.67 

1,194.4? 

.0946 

10.56 

659.7 

40 

267.12 

926.47 

1,195.41 

.0994 

1  0.06 

628.2 

42 

270.07 

924.36 

1,196.31 

.1041 

9-59 

599-3 

44 

272.91 

922.32 

1,197.18 

.1088 

9.18 

573-7 

46 

275-65 

920.36 

1,198.01 

-"34 

8.82 

550-4 

48 

278.30 

918.47 

1,198.82 

.n8[ 

8-47 

529.0 

5° 

280.85 

916.63 

1,199.60 

.1227 

8.15 

508-5 

52 

283.33 

914.86 

1,200.35 

.1274 

7.85 

490.1 

54 

285.72 

913-13 

I,20I.O9 

.1320 

7-58 

472-9 

56 

288.05 

911.46 

1,  20I.8O 

.1366 

7-32 

457-0 

58 

290.32 

909.83 

1,202.49 

.1411 

7.08 

442.4 

MECHANICAL   DRAFT. 


Table  No.  6.  —  Properties  of  Saturated  Steam. — Concluded. 


• 

2 

3 

4 

5 

6 

7 

60 

292.52 

908.25 

,203.16 

•1457 

6.86 

428-5 

62 

294.66 

906.70 

,203.81 

.1502 

6.66 

415.6 

64 

296.75 

905.20 

,204.45 

•1547 

6.46 

403.5 

66 

298.79 

903-73 

,205.07 

.1592 

6.28 

392.1 

68 

300.78 

902.30 

,205.68 

.1637 

6.10 

381.3 

70 

302.72 

900.90 

,206.27 

.1682 

5-95 

371-2 

72 

304.62 

899-53 

,206.85 

.1726 

5.80 

361.7 

74 

306.47 

898.19 

,20741 

.1770 

5-65 

352.6 

76 

308.29 

896.88 

,207.97 

.1814 

5-51 

344-1 

78 

310.07 

895.59 

,208.51 

.1858 

5-34 

336.0 

80 

311.81 

894.33 

,209.04 

.igOl 

5.26 

328.3 

82 

3I3-52 

893.09 

,209.56 

.1945 

S-M 

320.9 

84 

3I5-I9 

891.88 

,210.07 

.1989 

5-03 

3*3-9 

86 

316.84 

890.69 

,210.58 

.2032 

4.92 

307.2 

88 

3i8-45 

889.52 

,211.07 

.2075 

4-82 

300.8 

90 

320.04 

888.38 

,211-55 

.2118 

4.72 

294.7 

92 

321.60 

887.25 

,212.03 

.2l6l 

4-63 

288.9 

94 

323-!3 

886.14 

,212.49 

.2204 

4-54 

283-3 

96 

324-63 

885.04 

,212-95 

.2245 

4.44 

278.0 

-98 

326.11 

883.97 

,213.40 

.2288 

4-37 

272.8 

100 

327-57 

882.91 

,213.85 

•2330 

4.29 

267.9 

J°5 

331-11 

880.34 

,214.93 

•2434 

4.11 

256-5 

no 

334-S2 

877.86 

,215.97 

•2538 

3-94 

246.0 

11S 

337-8i 

87547 

,216.97 

.2640 

3-79 

236.3 

120 

340.99 

873-I5 

,217.94 

•2743 

3-65 

227.6 

I25 

344-07 

870.91 

,218.88 

•2843 

3-52 

219.7 

130 

347.06 

868.73 

,219.79 

.2942 

3-40 

212.3 

i35 

349-95 

866.62 

,220.68 

.3040 

3-29 

205.4 

140 

352-77 

864.57 

,221.53 

•3  '39 

3-'9 

199.0 

MS 

355-5° 

862.57 

,222.37  . 

•3239 

3-°9 

193.0 

150 

358.16 

860.62 

,223.18 

•3340 

2-99 

187.5 

1  60 

363.28 

856.87 

,224.74 

•3521 

2.84 

177-3 

170 

368.16 

853-29 

,226.23 

•3709 

2.69 

168.4 

180 

372-82 

849.87 

,227.65 

.3889 

2-57 

160.4 

190 

377-29 

846.58 

,229.01 

.4072 

2.45 

153-4 

200 

38i.57 

843-43 

,230.32 

.4250 

2-35 

147.1 

250 

401.07 

831.22 

,235-73 

.5464 

1-83 

114. 

300 

418.22 

819.61 

,240.74 

.6486 

i-54 

96. 

35° 

431.96 

810.69 

,244.58 

•7498 

i-33 

83- 

400 

444.92 

800.20 

,249.09 

.8502 

1.18 

73- 

I2  MECHANICAL   DRAFT. 

Specific  Heat.  —  The  specific  heat  of  saturated  steam  is  0.305  referred  to 
water  as  a  standard ;  that  is,  it  requires  only  0.305  as  much  heat  to  raise  the 
temperature  of  a  given  weight  of  saturated  steam  through  one  degree  as  would 
be  necessary  to  raise  the  same  weight  of  water  through  the  same  increment. 
Properly,  this  value  of  0.305  is  the  specific  heat  of  the  water  and  its  saturated 
vapor  combined.  The  specific  heat  of  gaseous  steam  is  0.475. 

Latent  and  Sensible  Heat.  —  When  water  is  heated  in  the  open  atmosphere 
its  temperature  gradually  increases  until  212°  is  reached.  Further  application  of 
heat  has  no  effect  in  raising  the  temperature  beyond  this  point,  but  the  water 
boils  and  passes  off  as  steam,  the  temperature  of  which  will  also  be  found  to  be 
212°.  Evidently,  then,  the  heat  applied  to  accomplish  the  vaporization  cannot 
be  measured  by  the  thermometer.  But  experiment  has  shown  that  in  the  evap- 
oration of  the  entire  volume  of  water  there  thus  disappears  about  five  and  a 
half  times  as  much  heat  as  is  required  to  raise  it  from  freezing  to  boiling  point. 
On  the  other  hand,  it  has  been  proven  by  experiment  that  upon  the  condensa- 
tion of  steam  there  is  relinquished  a  large  quantity  of  heat  of  which  the  ther- 
mometer gave  no  indication,  and  that  this  amount  is  exactly  equal  to  that  which 
disappeared  in  the  process  of  vaporization.  Because  of  the  hidden  character 
of  this  heat  it  is  known  as  latent  heat,  and  is  measured  in  thermal  units,  while 
that  indicated  by  the  thermometer  is  designated  as  sensible  heat.  The  sum  of 
the  heat  units  in  the  water  and  in  the  steam  generated  therefrom  is  known  as 
the  total  heat. 

The  total  heat  required  in  the  formation  of  steam  is  expended  in  three  ways. 

1.  In  raising  the  temperature  of  the  water  to  the  boiling  point.     This  would 
be  exactly  measured  by  the  sensible  heat  if  the  specific  heat  of  water  was  con- 
stant.    The  total  heat  of  the  water  in  heat  units  varies  slightly,  however,  from 
the  temperature  in  degrees  as  indicated  by  the  thermometer.     This  relation  is 
clearly  shown  in  Table  No.  3. 

2.  In  the  work  done  in  transforming  the  water  into  steam.     This  is  distinctly 
an  internal  work,  and  consists  in  separating  the  water  particles  and  establishing 
a  repulsive  action  between  them. 

3.  In  the  additional  work  of  overcoming  the  incumbent  pressure  of  the  sur- 
rounding atmosphere  so  that   enlargement    of   volume  may  take   place.     This 
work  is  entirely  external. 

An  analysis  of  the  heat  and  work  expenditures  in  increasing  the  temperature 
of  one  pound  of  water  from  32°  to  the  boiling  point,  and  transforming  it  into 
steam  of  212°  temperature  under  atmospheric  pressure,  is  displayed  in  Table 
No.  7,  from  which  the  relative  amount  of  heat  expended  in  each  of  the  three 
ways  is  rendered  evident. 


MECHANICAL    DRAFT. 


Table  No.  6,  in  which  are  given  the  properties  of  saturated  steam,  taken  in 
connection  with  Table  No.  3,  which  gives  the  number  of  heat  units  in  the  water 
for  any  given  temperature,  are  of  great  convenience  in  all  calculations  relating  to 
steam-boiler  performance. 

Table  No.  7.  —  Heat  and  Work  Required  to  Generate  One  Pound  of   Saturated  Steam 
at  2120  from  Water  at  320. 


DISTRIBUTION  OF  HEAT. 

Units  of 
Heat. 

Mechanical  Equiva- 
lent, in  Foot-  Pounds. 

The  Sensible  Heat. 
i.     To  raise  the  temperature  of  the  water  from  32°  to  212° 

180.9 

140,740 

The  Latent  Heat. 

2.     In  the  formation  of  steam 
3.     In  expansion  against  the  atmospheric  pressure 

Total  heat  and  work 

894.0 

71-7 

695,532 

55.783 

1,146.6 

892,055 

Flow  of  Steam.  —  When  steam  is  discharged  into  the  atmosphere  the  velocity 
of  outflow  at  constant  density  (when  the  absolute  pressures  are  greater  than  1.73 
times  the  atmospheric  pressure)  is  as  given  in  the  accompanying  Table  No.  8. 

Table  No.  8.  — Outflow  of  Steam  into  the  Atmosphere. 


Absolute 
Initial 
Pressure 
per  sq.  in. 

Pounds. 

Velocity 
of 
Outflow  at 
Constant 
Density. 
Feet  per 
Second. 

Actual 
Velocityof 
Outflow, 
Expanded 
Feet  per 
Second. 

|     Horse 
Discharge  :  Power  per 
per  sq.  in.  i  sq.  in.  of 
of  Orifice   j  Orifice  if 
per  min.    i   H.P.=3o 
Ibs.  per  hr. 
Pounds.           H.P. 

Absolute 
Initial 
Pressure 
per  sq.  in. 

Pounds. 

Velocity 

Outflow  at 
Constant 
Density. 
Feet  per 
Second. 

Actual 
Velocityof 
Outflow, 
Expanded 
Feet  per 
Second. 

Discharge 
per  sq.  in. 
of  Orifice 

Pounds. 

Horse 
Power  per 
sq.  in.  of 
Orifice  if 
H.P.  =30 
Ibs.  per  hr. 
HP. 

25-37 

863 

1,401 

22.8l              45.6 

90. 

895 

1,454 

77-94         155-9 

33- 

867 

I,408 

26.84         53-7 

IOO. 

898 

M59 

86.34         172.7 

40. 

874 

1,419 

35-18 

70.4 

11  5-    , 

902 

i,466 

98.76         197.5 

50- 

880 

1,429 

44.06 

88.1 

'35- 

906 

1,472 

115.61         231.2 

60. 

885 

J.437 

52-59 

105.2 

'55- 

910 

1,478 

132.21         264.4 

70. 

889 

J.444 

61.07 

122.  1 

165. 

9I2 

1,481 

140.46 

280.9 

75- 

89I 

1.447 

65.30 

130.6 

215. 

919 

i,493 

181.58         363.2 

The  external  pressure  per  square  inch  has  been  taken  as  that  existing  under  the 
standard  atmospheric  pressure  of  29.921  inches  of  mercury,  —  namely,  14.7 
pounds  absolute,  —  while  the  ratio  of  expansion  in  the  nozzle  itself  has  been 
taken  as  1.624. 

When  steam  flows  through  pipes  its  velocity  is  necessarily  decreased  by  the 


MECHANICAL   DRAFT, 


friction  engendered.  The  resistances  presented  by  bends  and  valves  seriously 
retard  the  flow.  The  following  Table  No.  9,  calculated  by  an  approximate  for- 
mula, gives  the  flow  of  steam  through  ordinary  steam  pipes  as  measured  in 
pounds  under  a  loss  of  one  pound  pressure. 

In  the  case  of  pipes  below  six  inches,  the  sizes  are  the  commercial  and  not 
the  actual  internal  diameters.  The  calculated  flow  has  in  each  instance  been 
figured  for  a  pipe  having  a  length  240  times  its  diameter.  The  flow  varies  as 
the  square  root  of  the  length  of  the  pipe. 

Table  No.  9.  —  Weight  of  Steam  in  Pounds  per  Minute  that  will  Flow  through  Pipes 
of  Given  Diameter  with  Loss  of  One  Pound  of  Pressure. 


Initial 
Gauge 
Pressure, 
in  Ibs.  per 
sq.  in. 

Diameter  of  Pipe,  in  Inches.     Length  of  Each  =  240  Diameters. 

X 

, 

*     |      » 

*K 

3 

4 

5 

6 

8 

,« 

I 

1.16 

2.07 

5-7 

IO.27 

1545 

25.38 

46.85 

77-3 

115.9 

211.4 

34I-I 

IO 

1.44 

2.57 

7-i 

12.72 

I9-I5 

31-45 

58.05 

95-8 

143.6 

262.0 

422.7 

20 

1.70 

3.02 

8-3 

14.94 

22.49 

36.94 

68.20 

II  2.6 

168.7 

307.8 

496.5 

30 

1.91 

3-40 

9-4 

16.84 

25-35 

41.63 

76.84 

126.9 

190.1 

346.8 

559-5 

40 

2.10 

3-74 

10.3 

18.51 

27.87 

45-77 

84.49 

139-5 

209.0 

381.3 

615-3 

5° 

2.27 

4.04 

II.  2 

20.01 

30-I3 

49.48 

91-34 

150.8 

226.O 

412.2 

665.0 

60 

2-43 

4-32 

II-9 

21.38 

32.19 

52-87 

97.60 

161.1 

241.5 

440.5 

710.6 

70 

2-57 

4.58 

12.6 

22.65 

34-iQ 

56.00 

103-37 

170.7 

255-8 

466.5 

752-7 

80 

2.71 

4-82 

13-3 

23.82 

35-87 

58.91 

108.74 

179-5 

269.0 

490.7 

791.7 

90 

2.83 

5-04 

13-9 

24.92 

37-52 

61.62 

"3-74 

187.8 

281.4 

5*3-3 

828.1 

100 

2-95 

5-25 

14-5 

25.96 

39-07 

64.18 

118.47 

195.6 

293-1 

534.6 

862.6 

1  20 

3.16 

5-63 

15-5 

27-85 

41-93 

68.87 

127.12 

209.9 

3H-5 

573-7 

925.6 

Steam-Pipe  Coverings.  —  The  condensation  of  steam  inside  an  unprotected 
steam  pipe  is  dependent  upon  the  temperature  of  the  steam  within,  the  tempera- 
ture of  the  air  without  and  the  velocity  of  movement  of  that  air.  Under  all 
ordinary  condition  the  condensation  is  so  great  as  to  warrant  considerable 
expenditure  for  its  prevention,  which  may  be  accomplished  to  a  greater  or  less 
extent  by  applying  to  the  exterior  of  the  pipe  proper  coverings  having  the 
minimum  facility  for  conveying  heat.  Such  coverings  depend  not  only  upon 
the  material  of  which  they  are  constructed, —  which  should  evidently  be  non- 
combustible, —  but  largely  upon  the  air  which  is  held  between  the  particles  of 
the  substance.  A  material  which  is  non-combustible  and  at  the  same  time  of  a 
porous  or  spongy  nature,  with  numerous  air  cells  or  spaces,  is  naturally  adaptable 
as  a  covering. 


MECHANICAL   DRAFT. 


The  relative  value  of  such  a  covering  is  most  readily  expressed  by  the  num- 
ber of  heat  units  it  will  transmit  under  given  conditions ;  the  lower  its 
conductivity,  the  higher  its  efficiency.  The  results  of  very  carefully  conducted 
tests  by  Mr.  C.  L.  Norton,  for  the  Massachusetts  Manufacturers'  Mutual  Fire 
Insurance  Company,  are  presented  in  Table  No.  10,  and  serve  to  indicate  the 
relative  values  of  familiar  steam-pipe  coverings. 

Table  No.   10.  —  Steam-Pipe  Coverings. 


Weight, 

Temperature  Corresponding 
to  10  Ibs.  Steam  Pressure. 

Temperature  Corresponding 
to  200  Ibs.  Steam  Pressure. 

Name  of 

Thickness,    j 

in 
Ounces 

Covering. 

Inches. 

per 

B.T.U.  Loss 

Ratio  of  Heat 

B.T.U.  Loss 

Ratio  of  Heat 

Square 

per  sq.  ft.  of      Loss  to  Loss 

per  sq.  ft.  of 

Loss  to  Loss 

Foot. 

Pipe 
per  Mhmte. 

from 
Bare  Pipe. 

Pipe 
per  Minute. 

from 
Bare  Pipe. 

Nonpareil  Cork. 

0.90 

21 

1.44 

0.232 

3-04 

0.254 

Magnesia.                             1.12 

24 

1.59                0.262 

340 

0.284 

Air  Cell  No.  i. 

1.  12 

23 

'         

3-58 

0.300 

Air  Cell  No.  2. 

1-25 

36 

1.58                 0.261 

3-40 

0.284 

Magnabestos. 

1.  12 

48 

2.32                 0.383 

3-84 

0.32I 

Fire  Felt. 

1.  00 

46 

2.40 

o-395 

3-99 

o-333 

Calcite. 

1.25 

29 





5.02 

0.423 

Bare  Pipe. 





6.06              i  .000 

11.96 

I.OOO 

CHAPTER    III. 
COMBUSTION. 

Definition.  —  Although  in  its  broadest  sense  the  term  "  combustion  "  applies  to 
all  forms  of  chemical  union,  in  its  common  acceptance  it  has  reference  only  to 
the  process  of  burning,  whereby  a  substance  unites  with  oxygen,  with  the  result- 
ing phenomena  of  light  and  heat.  A  combustible  may,  therefore,  be  defined  as  a 
substance  capable  of  combining  rapidly  with  oxygen  so  as  to  produce  light  and 
heat,  while  oxygen  may  be  classed  as  a  supporter  of  combustion. 

Carbon.  —  Of  all  combustibles  carbon  is  the  most  widely  distributed,  and 
readily  obtained  in  nature.  Because  of  its  abundance  as  a  constituent  of  coal, 
wood,  peat,  mineral  oil  and  natural  gas,  these  substances  are  almost  exclusively 
adopted  as  fuels.  To  these  may  be  added  coke,  charcoal  and  fuel  gas,  which 
are  produced  by  special  processes  from  these  natural  substances.  Carbon  itself 
is  an  infusible,  non-volatile  solid,  of  which  three  distinct  modifications  occur ; 
viz.,  (i)  diamond,  (2)  plumbago,  or  graphite,  (3)  charcoal,  or  lampblack.  Among 
natural  fuels,  —  that  is,  those  not  prepared  by  artificial  means,  —  anthracite 
coal  most  nearly  approaches  to  the  condition  of  pure  carbon,  and  is  to  be  classed 
between  graphite  and  charcoal. 

Oxygen.- — Although  oxygen,  the  universal  supporter  of  combustion,  as  here 
defined,  is  the  most  abundant  of  all  natural  substances,  it  never  exists  by  itself 
in  nature,  but  always  in  association  with  some  other  substance.  Thus  it  is  that 
as  a  constituent  of  atmospheric  air  it  is  associated  with  the  inert  gas,  nitrogen ; 
the  relative  proportions  of  the  two  gases  in  pure  air  remaining  practically  con- 
stant under  all  conditions. 

As  determined  by  recent  investigation,  pure  air  is  composed,  by  volume,  of  — 

Oxygen          .         .         .         .         ....         0.213  parts 

Nitrogen       .         .         .         .  .         .         ,         0-787      " 

i.ooo 
and  by  weight  of  — 

Oxygen          .         .'.... 0.236  parts 

Nitrogen       ...         .         .         .         .          .         0.764     " 


MECHANICAL   DRAFT.  17 

In  nature,  however,  this  proportionate  composition  of  pure  air  is  slightly  affected 
by  the  presence  of  aqueous  vapor,  carbonic  acid  and  other  impurities.  Unless 
extreme  accuracy  is  desired,  it  is  usually  convenient  to  consider  the  atmos- 
phere as  composed  of  one  volume  of  oxygen  and  four  volumes  of  nitrogen.  Air 
is,  however,  but  a  mechanical  mixture  of  the  two  gases,  and  the  oxygen  is,  there- 
fore, free,  without  chemical  dissociation,  to  leave  the  nitrogen  and  unite  with 
other  substances,  which  it  does  with  great  avidity  under  favorable  circumstances. 
In  its  independent  state,  oxygen  is  colorless,  tasteless  and  slightly  heavier  than 
air,  in  the  proportion  of  i  to  1.1056. 

The  Atomic  Theory.  —  A  clear  understanding  of  the  atomic  theory  is  neces- 
sary to  a  full  comprehension  of  the  principles  of  combustion.  This  theory, 
which  has  been  developed  through  years  of  investigation,  is  now  universally 
accepted  as  the  explanation  of  all  chemical  phenomena. 

By  experiment  it  has  been  demonstrated  that  all  chemical  combinations  be- 
tween elementary  substances  are  made  in  definite  and  invariable  proportions. 
For  instance,  if  hydrogen  and  oxygen  be  mixed  and  caused  to  form  water,  as 
already  described,  it  will  be  found  that  the  entire  amount  of  these  two  gases 
will  be  utilized  and  enter  into  combination,  only  when  they  originally  existed  in 
the  exact  proportion  of  two  volumes  of  hydrogen  to  one  volume  of  oxygen. 
Observation  also  proves  that  if  this  water  is  maintained  in  its  gaseous  condition 
it  will  occupy  only  the  space  of  two  volumes,  although  for  its  production  a  total 
of  three  volumes  was  supplied. 

Two  volumes  of  hydrogen  and  one  and  a  half  volumes  of  oxygen  cannot  be 
made  to  chemically  combine  to  form  the  compound  water,  for  the  hydrogen  will 
unite  with  only  its  proportional  quantity,  leaving  the  extra  half-volume  of  oxygen 
unassociated.  No  matter  how  large  or  how  small  these  volumes  may  be,  the  same 
relation  holds.  It  is,  therefore,  reasonable  to  suppose  that,  if  the  smallest  con- 
ceivable particle  of  oxygen  be  brought  into  union  with  two  of  the  smallest 
conceivable  particles  of  hydrogen,  there  will  be  the  same  result  and  a  minute 
particle  of  water  will  be  formed. 

These  minute  particles,  the  smallest  in  which  elementary  substances  may  be 
conceived  to  enter  into  combination  with  each  other,  are  called  atoms,  while  the 
individual  particles  resulting  from  their  union  are  known  as  molecules.  From  the 
above  reasoning  it  would  appear  probable  that  equal  volumes  of  the  elementary 
gases,  at  least,  contain  the  same  number  of  atoms,  and,  therefore,  that  the  atoms 
are  of  equal  size.  Although  attempts  have  been  made  to  calculate  the  probable 
dimensions  of  these  atoms,  we  have  no  direct  knowledge  as  to  their  size. 

Chemists  have  adopted  as  designating  symbols  for  the  various  elements  the 
initials  of  their  names,  followed,  when  necessary  for  distinction,  by  a  succeeding 


iS 


MECHANICAL   DRAFT. 


letter.  Thus  hydrogen  is  designated  by  H  and  oxygen  by  O.  The  compound 
water,  formed  by  the  chemical  union  of  two  atoms  of  hydrogen  and  one  atom  of 
oxygen,  can,  therefore,  be  simply  represented  by  H2O,  the  suffix  "2"  being 
employed  to  indicate  the  presence  by  volume  of  twice  as  much  hydrogen  as 
oxygen. 

Upon  the  assumption  that  the  atoms  are  of  equal  size,  the  determination  of 
the  relative  weights  of  equal  volumes  of  these  two  gases,  under  the  same  pres- 
sure and  temperature,  is  equivalent  to  determining  the  relative  weights  of  the 
atoms  themselves  ;  that  is,  their  atomic  weights.  The  weight  of  hydrogen,  which 
is  the  lightest  of  all  known  substances,  is  taken  as  unity,  the  relative  weight  of 
oxygen  being  16.  That  is,  a  given  volume  of  oxygen  weighs  16  times  as  much 
as  an  equal  volume  of  hydrogen. 

The  symbol  H2O,  therefore,  reveals  still  another  fact  as  to  the  composition  of 
water;  namely,  that  2  atoms  of  hydrogen,  weighing  relatively  2x1  =  2,  are 
combined  with  i  atom  of  oxygen  weighing  16.  In  other  words,  that  by  weight, 
water  is  composed  of  2  parts  of  hydrogen  and  16  parts  of  oxygen;  or  more 
simply,  that  the  ratio  of  the  hydrogen  to  the  oxygen  is  as  i  to  8. 

But  in  this  process  of  combination  it  has  already  been  shown  that  the  two 
volumes  of  hydrogen  and  one  volume  of  oxygen  unite  to  form  only  two  volumes 
of  water  in  its  gaseous  state,  which  two  volumes  represent  the  space  originally 
occupied  by  the  hydrogen.  Hence  it  is  evident  that  the  compound  now  weigh- 
ing 1 8  occupies  the  same  space  as  an  amount  of  hydrogen  weighing  2,  and  that  its 

relative  density  is  —  =  9.    In  other  words,  gaseous  water  of  given  temperature 

and  pressure  weighs  nine  times  as  much  as  an  equal  volume  of  hydrogen  under 
the  same  conditions. 

The  common  elementary  substances  entering  into  the  composition  of  fuel, 
with  their  symbols  and  atomic  weights  in  round  numbers,  are  given  in  Table 
No.  ii. 

Table  No.  n.  —  Symbols    and   Atomic  Weights  of  Elementary   Substances    concerned 

in  Combustion. 


Name. 

Symbol. 

Atomic  Weight. 

Hydrogen. 

H. 

I 

Carbon. 

C. 

12 

Nitrogen. 

N. 

M 

Oxygen. 

O. 

16 

Sulphur. 

s. 

32 

MECHANICAL    DRAFT.  19 

Union  of  Carbon  and  Oxygen.  —  Many  elements  enter  into  chemical  com- 
bination with  each  other  in  more  than  one  proportion.  This  is  true  of  carbon 
and  oxygen.  If  a  piece  of  carbon,  heated  to  incandescence,  be  placed  in  a 
sufficient  volume  of  oxygen  or  air,  each  atom  of  the  carbon  will  unite  with  two 
atoms  of  the  oxygen,  to  form  a  compound  known  as  carbonic  acid,  or  carbonic 
dioxide,  the  symbol  of  which  is  CO2 ;  the  process  being  indicated  by  the  formula 
C-f-2O  =  CO2.  No  matter  how  plenteous  the  oxygen,  it  cannot  be  made  to 
enter  into  combination  with  the  carbon  in  a  proportion  greater  than  two  atoms  of 
oxygen  to  one  atom  of  carbon. 

This  gas  is,  therefore,  evidently  a  product  of  complete  combustion,  there 
having  been  a  full  supply  of  oxygen.  As  is  shown  by  Table  No.  n,  the  single 
atom  of  carbon  weighs  12  relatively  to  each  atom  of  oxygen  which  weighs  16; 
that  is,  the  compound  consists  by  weight  of  12  parts  of  carbon,  and  2  x  16  =  32 
parts  of  oxygen. 

Carbonic  acid  gas  is  transparent  and  colorless,  about  one  and  a  half  times 
heavier  than  air,  and  of  a  slightly  acid  taste  and  smell.  It  is  incombustible, 
being  already  the  product  of  complete  combustion,  and,  although  not  directly 
poisonous,  is  neither  a  supporter  of  animal  life  nor  of  combustion. 

If,  in  turn,  this  gas,  without  the  accompaniment  of  sufficient  oxygen,  be  brought 
into  contact  with  incandescent  carbon,  it  will  be  deprived  of  one-half  its  oxygen, 
each  atom  of  oxygen  thus  released  uniting  with  an  atom  of  carbon  to  form  a 
new  compound  known  as  carbonic  oxide,  with  the  symbol  CO.  The  process  of 
combination  may  be  symbolically  expressed  thus:  CO2-j- C  =  2CO,  showing 
that  not  only  is  the  new  compound  formed  by  union  of  carbon  with  the  released 
oxygen,  but  that  the  carbonic  acid  thus  deprived  of  its  oxygen  is  thereby  also 
reduced  to  carbonic  oxide.  The  relative  weights  of  the  elements  of  this  com- 
pound are,  evidently,  carbon  =  12,  and  oxygen  =  16. 

This  gas  is  slightly  lighter  than  air,  transparent,  colorless  and  practically 
odorless,  and  is  destructive  to  animal  life,  being,  in  fact,  a  direct  poison.  It  is 
not  a  supporter  of  combustion,  but,  being  the  product  of  imperfect  combustion, 
is  itself  a  combustible  and  may  be  readily  burned  in  the  air.  Such  being  the 
case,  we  should  expect  that  the  process  of  burning  —  which  has  already  been 
denned  as  the  rapid  chemical  union  of  a  combustible  with  oxygen  —  would 
result  in  an  accession  of  oxygen  to  the  carbonic  oxide.  Experiment  will  prove 
this  to  be  true,  the  product  being  carbonic  acid,  the  same  compound  already 
shown  to  be  the  result  of  complete  combustion.  Symbolically,  the  process  is 
expressed  by  CO  -f-  O  =  CO2.  In  tabular  form,  the  general  properties  of 
carbonic  oxide  and  carbonic  acid  are  shown  in  Table  No.  12,  upon  the  succeed- 
ing page. 


MECHANICAL   DRAFT. 


Table  No.  12.  —  Properties  of  Carbonic   Oxide  and  Carbonic  Acid. 


Name. 

Symbol. 

COMPOSITION. 

By  Weight.  - 

Percentage. 

Carbon. 

Oxygen. 

Total. 

Carbon. 

Oxygen. 

Total. 

Carbonic  oxide, 
Carbonic  acid, 

CO 
CO2 

" 

16 
3- 

28 
44 

42.86 

27.27 

57-14 

72-73 

100 
100 

Combustion  of  Fuel.  —  The  two  elements  contributing  most  largely  to  the 
economic  value  of  any  fuel,  as  measured  by  its  heating  power,  are  carbon  and 
hydrogen.  These  elements  exist  in  fuels  either  combined,  or,  upon  the  applica- 
tion of  heat,  associate  themselves  in  a  series  of  complex  compounds  known  as 
hydro-carbons,  the  simplest  of  the  list  of  some  fifty  being  marsh  gas,  represented 
by  the  symbol  CH4.  Such  portion  of  the  carbon,  or  hydrogen,  as  does  not  thus 
enter  into  combination,  and  for  which  there  exists  in  the  entire  substance  no 
further  material  for  combination,  is  designated  as  fixed. 

Besides  these  primary  elements,  fuels  usually  contain  small  amounts  of  oxygen, 
nitrogen  and  sulphur,  together  with  a  certain  percentage  of  incombustible  matter 
which  remains  as  ash  after  the  process  of  combustion  is  complete.  The  phe- 
nomena attendant  upon  the  combustion  of  ordinary  fuels  are,  therefore,  much 
more  complex  than  those  resulting  from  the  combustion  of  carbon  alone. 
Although  it  must  be  evident  that  fuels  of  the  same  general  character  vary  con- 
siderably in  the  proportions  of  their  constituents,  their  relative  average  elemen- 
tary composition  by  weight  is  substantially  as  shown  in  Table  No.  13.  The 
results  there  given  are  those  determined  by  ultimate  analysis.  For  the  general 
purposes  of  comparison  of  fuels,  the  method  of  proximate  analysis,  whereby  only 
the  relative  percentages  of  carbon,  volatile  matter,  ash  and  moisture  are  ascer- 
tained, is  sufficiently  refined. 

Obviously,  owing  to  the  conditions  under  which  combustion  takes  place,  it  is- 
impossible  to  determine  in  detail  the  exact  order  of  the  process.  It  is  certain, 
however,  that  the  final  results  of  perfect  combustion  of  ordinary  fuel  should  be 
carbonic  acid  gas  (CO2),  water  (H2O),  nitrogen  (N),  and  possibly  a  little  sulphur- 
ous acid  (SO2).  The  process  may  be  outlined  as  follows  :  If,  for  instance,  coal 
of  bituminous  character  be  thrown  upon  a  glowing  fire,  the  heat  first  volatil- 
izes and  frees  the  hydro-carbons,  at  comparatively  low  temperature.  These 
inflammable  gases  are  thereupon  immediately  ignited,  and  by  the  heat  thus  pro- 
duced assist  in  bringing  the  remainder  of  the  coal  to  a  state  of  incandescence. 
The  burning  of  the  hydro-carbons  is  indicative  of  their  union  with  oxygen, 
whereby  these  compounds  are  broken  up  and  new  combinations  of  a  simpler 


MECHANICAL    DRAFT. 


character  are  formed.  The  three  elements  thus  presented  for  combination  are 
carbon,  hydrogen  and 'oxygen.  If  the  supply  of  oxygen  is  sufficient,  the  carbon 
leaves  the  hydrogen  with  which  it  has  been  associated,  and  unites  with  the 

Table  No.   13.  —  Composition  of  Fuels. 


DESCRIPTION. 

Carbon. 

Hydrogen. 

Oxygen. 

Nitrogen. 

Sulphur. 

Ash. 

ANTHRACITES. 

France, 

90.9 

1.47 

1-53 

I.OO 

O.So 

4-3 

Wales, 

91.7 

3-78 

I.30 

1.  00 

0.72 

J-5 

Rhode  Island, 

85-0 

3-71 

2-39 

I.OO 

0.90 

7.0 

Pennsylvania, 

78.6 

2-5 

*-7 

0.8 

0.4 

14.8 

SEMI-BITUMINOUS. 

Maryland, 

So.O 

5-° 

2-7. 

I.I 

1.2 

8-3 

Wales, 

88.3 

4-7 

0.6 

1.4 

1.8 

3-2 

BITUMINOUS. 

Pennsylvania, 

75-5 

4-93 

I2-35 

1.  12 

.10 

5-o 

Indiana, 

69.7 

5.10 

19.17 

1-23 

•3° 

3-5 

Illinois, 

61.4 

4-87 

35-42 

I.4I 

.20 

5-7 

Virginia, 

57-o 

4.96 

26.44 

1.70 

•5° 

8.4 

Alabama, 

53-2 

4.81 

32.37 

1.62 

•3° 

6-7 

Kentucky, 

49.1 

4-95 

4!-!3 

1.70 

.40 

7.2 

Cape  Breton, 

67.2 

4.26 

20.  1  6 

1.07 

.21 

6.1 

Vancouver's  Island, 

66.9 

5-32 

8.76 

1.02 

2.20 

15.8 

Lancashire  gas-coal, 

80.  i 

5-5 

0     _ 

2.1 

i-5 

2-7 

Boghead  cannel, 

63.1 

8.9 

7-0 

0.2 

I.O 

19.8 

LIGNITES. 

California  brown  coal, 

49-7 

3-78 

30.19 

1.0 

i-53 

13.8 

Australian  brown  coal, 

73-2 

4.71 

12-35 

I.  II 

0.63 

8.0 

PETROLEUMS. 

Pennsylvania,  crude, 

84.9 

T3-7 

1.4 







Caucasian,  light, 

86.3 

13-6 

O.I 







Caucasian,  heavy, 

86.6 

12-3 

I.I 







Refuse, 

87.1 

11.7 

1.2 







oxygen  to  form  carbonic  acid,  an  evidence  that  combustion  is  complete.  The 
liberated  hydrogen  also  unites  with  oxygen,  if  a  sufficiency  is  present,  and  forms 
water,  which,  being  at  a  high  temperature,  is  maintained  in  its  gaseous  con- 
dition. If,  upon  dissociation,  a  portion  of  the  carbon  which  is  liberated  in 
incandescent  particles  does  not  immediately  meet  with  its  complement  of  oxygen, 
it  is  liable  to  become  cooled  to  such  an  extent  by  the  surrounding  gases  that, 


22  MECHANICAL   DRAFT. 

when  it  reaches  an  abundance  of  oxygen,  its  temperature  will  be  too  low  to 
permit  of  chemical  union.  It  will,  therefore,  pass  off  as  unconsumed  and 
visible  carbon,  in  the  form  of  smoke. 

By  the  time  the  coal  has  become  incandescent  all  of  the  hydro-carbons  will 
have  been  expelled,  and  the  carbon  will  be  in  a  condition  to  enter  into  combina- 
tion with  the  oxygen  of  the  air,  or  of  any  surrounding  carbonic  acid.  If  oxygen 
is  present  in  excess,  the  product  will  be  carbonic  acid;  but  if  carbonic  acid  be 
brought  into  contact  with  the  glowing  coal,  carbonic  oxide  will  be  the  result. 
This,  in  turn,  will  burn  to  carbonic  acid,  if  only  supplied  with  sufficient  air. 

In  consideration  of  the  number  and  great  variety  of  interstices  existing  between 
the  lumps  of  coal,  and  of  the  various  stages  of  combustion  to  which  different 
portions  of  the  fuel  have  attained,  it  is  evident  that  in  their  passage  through  the 
fire  many  changes  must  take  place  in  the  composition  of  the  gases.  Association 
and  dissociation  must  follow  in  rapid  succession ;  at  one  instant  an  atom  of 
carbon  may  be  combined  with  two  atoms  of  oxygen  to  form  carbonic  acid,  while 
in  the  next  it  may  have  lost  one  of  its  atoms  of  oxygen  and  have  been  reduced 
to  carbonic  oxide,  which,  in  turn,  may  come  in  contact  with  a  sufficiency  of 
oxygen  to  again  form  carbonic  acid.  In  just  which  combination  the  carbon  and 
oxygen  shall  leave  the  fire  and  pass  to  the  chimney  must  depend  upon  the  tem- 
perature of  the  gases  and  the  proportion  of  oxygen  at  hand. 

Air  Required  for  Combustion.  —  The  definite  proportions  in  which  oxygen 
unites  with  hydrogen  and  carbon  to  form  water  and  carbonic  acid  —  the  results 
of  perfect  combustion  —  have  already  been  shown.  Expressed  in  pounds,  one 
pound  of  hydrogen  requires  eight  pounds  of  oxygen  for  its  complete  combustion ; 
for  the  atomic  weights  are  respectively  H  =  i  and  O  =  16,  and  the  composition 
of  water,  which  is  the  product  of  perfect  combustion  of  hydrogen  and  oxygen, 
is  symbolically  indicated  by  H2O.  Hence,  substituting  atomic  weights  for  sym- 
bols, H2O  =  2  +  16,  and  H2:O::2  :  16  =  H2:O  ::  i  :8. 

For  the  complete  combustion  of  one  pound  of  carbon  there  are  required  2^3 
pounds  of  oxygen,  for,  by  atomic  weights,  C  =  12  and  O  =  16  ;  hence,  CO2  = 
12  -f-  (2x16)  and  C  :  O2::  12  :32  =C  :  O2  ::  i  :  2^i. 

Air  consists,  by  weight,  of  0.236  parts  oxygen ;  therefore,  the  amount  of  air 
required  for  the  combustion  of  one  pound  of  carbon  must  be  the  amount  that 
would  contain  2^3  pounds  of  oxygen ;  that  is,  2^  -f-  0.236  =  1 1.3  pounds. 

In  Table  No.  14  are  presented  the  principal  data  regarding  oxygen,  nitrogen 
and  the  elements  of  the  common  combustibles,  together  with  the  amount  of  oxy- 
gen and  air  required  for  each  as  calculated  in  the  manner  just  described. 

As  already  shown  in  Table  No.  13,  oxygen  enters  to  a  certain  extent  into  the 
original  composition  of  all  fuels.  In  the  process  of  combustion,  this  oxygen 


MECHANICAL    DRAFT. 


unites  with  its  equivalent  of  hydrogen,  which  is  thus  rendered  inert,  so  far  as 
combination  with  extraneous  oxygen  is  concerned.  In  calculation,  therefore, 
this  quantity  of  hydrogen  is  disregarded,  and  there  are  left  to  be  considered  only 
the  remaining  carbon  and  hydrogen. 

Table  No.  14.  —  Combustion  Data. 


COMBUSTIBLE. 

PRODUCT  OF  COMBUSTION. 

REQUIRED  PER  POUND 
OF  COMBUSTIBLE. 

if 

Density 
or  Weight 

14 

Density 
or  Weight 

Oxygen. 

Air. 

NAME. 

Symbol. 

u^ 

of  One 
Cubic  Ft. 

NAME. 

Symbol. 

J? 

of  One 
Cubic  Ft. 

11 

Pounds. 

£JS 
-  3 

Pounds. 

Pounds. 

Pounds. 

Cubic 
Feet 
at  62°. 

Oxygen, 

0 

16 

0.08928 

Nitrogen,              N     |    14 

0.07837 

Hydrogen,            H 

i 

0.00559 

Water, 

H2O 

18 



8.00 

33-9 

444 

Carbon,                 C 

12 



Carbonic  ox. 

CO 

28 

0.07806 

«-33 

5-7 

.    75 

Carbon,                 C 

12 



Carbonic  acid 

C02 

44 

0.12341 

2.67 

"•3 

148 

Carbonic  ox. 

CO 

28 

0.07806 

Carbonic  'acid 

C02 

44 

0.12341 

0.57 

2.41       32 

(  Water, 

H2O 

18 



) 

Marsh  gas, 

CH4 

16 

0.04464 

]  Carbonic  ) 
C       acid,      \ 

C02 

44 

0.12341 

^4.00 

16.9 

222 

(  Water, 

H2O 

18 



i 

Olefiant  gas, 

C2H4 

28 

0.07809 

]  Carbonic  ) 
(       acid,      ( 

C02 

44 

0.12341 

h 

14-5 

100 

Sulphur,                S 

32 



Sulphurous  I 
acid,        \ 

S02 

64 

0.17860 

1.  00 

4.25 

56 

The  method  of  calculation  of  the  amount  of  air  necessary  for  the  combustion 
of  ordinary  coal  can  best  be  explained  by  means  of  an  example  based  upon  the 
known  composition  of  a  certain  fuel,  as,  for  instance,  that  of  the  Maryland  semi- 
bituminous  coal,  given  in  Table  No.   13.     For    simplicity  these  proportionate 
figures  are  here  given  in  pounds  instead  of  in  per  cent. 

Carbon        .         .         .         ....         .         80.0  pounds. 

Hydrogen   .         .         .         .         , ".      .         .  5-° 

Oxygen        .         .         ....         .         .  2.7 

Nitrogen     .         .          .         .  •  ;    .         .         .  i.i        " 

Sulphur 1.2        " 

Ash 8.3 

The  nitrogen  is  inert,  the  sulphur,  because  of  the  small  amount  in  which  it 
is  present,  may  be  disregarded,  and  the  ash,  being  incombustible,  has  no  effect 
on  the  result. 


24  MECHANICAL    DRAFT. 

We  may,  therefore,  estimate  as  follows  for  the  amount  of  oxygen  required: 
The  2.7  pounds  of  oxygen  will  render  inert  '-  =  0.3375  pounds  of  hydrogen, 

o 

for  it  will  directly  combine  with   that   amount.     The   constituents   to   be   con- 
sidered thus  become  — 

Carbon         .         .         .         .          .         .          .80  pounds. 

Hydrogen  5.0  —  0.3375=    ....     4.6625        " 

84.6625  pounds, 
and  their  requirements  in  the  way  of  oxygen  will  be  — 

Carbon  80  x  2^3  =  2I3-33  pounds. 

Hydrogen  4.6625  x  8  =  .         .         .  37-3Q        " 

250.63  pounds. 

The  weight  of  air  containing  the  above  quantity  of  oxygen  is  •*  '  =  1061.57 
pounds.  The  quantity  required  per  pound  of  combustible  will,  therefore,  be 

—    '      =  12.54  pounds.     As  these  constituents   are  part  of  a  quantity  of  coal 
84.6625 

weighing  100  pounds,  when  its  elementary  moisture  is  included,  the  amount  of 

air  necessary,  per  pound  of  coal,  is  -1-    *'5?  =  10.62  pounds. 

100 

For  approximate  calculation  of  the  weight  of  air  required  for  combustion  the 
following  formula  may  be  used :  — 

Weight  of  air=  I2C  +  36  (H  —  9). 

In  this  equation,  the  weights  of  carbon,  hydrogen  and  oxygen  are  represented 
respectively  by  their  symbols,  C,  H  and  O,  the  amount  of  hydrogen  rendered 
inert  by  the  oxygen  in  the  fuel  is  allowed  for,  and  the  proportion  of  oxygen  and 
nitrogen  in  the  atmosphere  is  taken  as  one  to  four. 

The  air  required  per  pound  of  fuel,  for  various  fuels  of  typical  composition,  as 
calculated  by  the  above  formula,  is  shown  in  Table  No. '15  ;  the  weights  of  the 
elements  and  of  the  air  being  given  in  pounds. 

As  is  evident  by  what  follows,  it  is  unnecessary,  for  practical  purposes,  to 
compute  with  great  exactness  the  weight  of  air  necessary  for  the  combustion  of 
fuel ;  for  the  excess  of  air  which  is  usually  supplied,  together  with  the  variable- 
ness in  the  composition  of  fuel,  renders  all  ordinary  calculations  somewhat 
approximate.  It  is,  therefore,  the  common  practice  to  estimate  the  approximate 
amount  required,  for  either  coke  or  coal,  at  12  pounds  per  pound  of  fuel. 


MECHANICAL    DRAFT. 

Table  No.   15.  —  Air  Required  for  Combustion  of  Fuels. 


Fuel. 

Weight  of  Given  Constituent  in  One  Pound  of  Fuel. 

Air  Required 
per  Pound 
of  Fuel. 
Pounds. 

Carbon. 

Hydrogen. 

Oxygen. 

CHARCOA 

L  —  From  wood, 

0-93 





II.6 

" 

From  peat, 

0.8o 





9.6 

COKE  — 

Good, 

0.94 





11.28 

COAL  — 

Anthracite, 

0.915 

0.035 

O.O26 

12.13 

" 

Dry  bituminous, 

0.87 

0.05 

0.04 

1  2.o6 

" 

Coking, 

0.85 

0.05 

O.o6 

"73 

Coking, 

0-75 

0.05 

0.05 

10.58 

" 

Cannel, 

0.84 

0.06 

0.08 

u.88 

<• 

Dry,  long-flaming, 

0.77 

0.05 

0.15                           10.32 

<; 

Lignite, 

0.70 

0.05 

0.20                                   9.30 

PEAT  — 

Dry, 

0.58 

O.06 

0.31                                   7.68 

WOOD  — 

Dry, 

0.50 



6.00 

MINERAL 

OIL, 

0.85 



15.65 

Air  for  Dilution.  —  The  preceding  calculations  of  air  supply  are  based  upon 
the  assumption  that  each  individual  atom  of  oxygen  in  the  air  comes  in  con- 
tact and  unites  with  its  proportion  of  hydrogen  or  carbon  in  the  fuel.  When  it 
is  considered  that  this  oxygen  is  intimately  united  with  about  four  times  its 
volume  of  nitrogen,  whereby  it  is  to  a  certain  extent  separated  from  the  fuel, 
and,  further,  that  the  variety  in  the  arrangement  of  the  fuel  and  the  passages 
through  it  affects  any  attempt  at  equal  distribution  of  the  air,  it  must  be  evident 
that  the  above  assumption  cannot  ordinarily  be  maintained  in  practice.  It, 
therefore,  usually  becomes  necessary  in  practice  to  furnish  sufficient  air  in 
excess  of  the  calculated  amount  to  insure  complete  combustion  in  all  parts  of 
the  furnace. 

Evidently,  the  amount  of  air  supplied  for  .dilution  must  vary  greatly  in  differ- 
ent cases.  This  is  clearly  shown  by  the  results  of  numerous  careful  tests 
of  different  boilers  by  Messrs.  Donkin  and  Kennedy.  In  each  case  the  volume 
of  air  supplied  was  determined  by  chemical  analysis  of  the  furnace  gases,  the 
results  of  which,  together  with  the  deductions  relating  to  the  amount  of  air 
supplied,  are  presented  in  Table  No.  16.  It  will  be  noted  that  the  dry  air  sup- 
plied, per  pound  of  coal,  ranges  from  16.1  pounds  to  40.7  pounds,  and  that  the 
corresponding  ratio  of  air  used  to  air  theoretically  required  ranges  from  1.56 
to  4.28  ;  that  is,  from  56  per  cent  to  328  per  cent  in  excess.  The  composition 
of  the  gases  is  given  by  weight. 


26  MECHANICAL   DRAFT. 

Table  No.  16.  —  Analysis  and  Calculations  Relating  to  Furnace  Gases  and  Air  Supply. 


3 

DUMBER 

OF  TEST 

I 

II 

III 

IV 

V 

VI 

VII 

VIII 

Per  cent  of  CO2, 

I5-I5 

13.00 

18.21 

11.71 

7.90 

10.44 

1  1.  CO 

M-95 

Per  cent  of  CO, 

2-59 

0.00 

0.24 

0.00 

o.oo 

0.28 

O.IO 

0.28 

Per  cent  of  O, 

6.46 

11.15 

7-55 

13-13 

17.00 

13-37 

13.20 

8.31 

Per  cent  of  N, 

75.80 

75-85 

74.00 

75.16 

75.10 

75-90 

75-70 

76.46 

Pounds  dry  air  per  pound  of  C, 

18.5 

27.6 

19.2 

30-7 

46.0 

33-i 

32-3 

23-3 

Pounds  dry  air  per  pound  of  coal, 

16.4 

24.4 

17.0 

27.2 

40.7 

29-3 

28.6 

20.6 

Ditto,  per  pound  pure  dry  coal, 

16.9 

25.2 

'7-5 

28.0 

42.2 

3°-3 

29.6 

21.2 

Pounds  dry  furnace  gases  per  pound  / 
pure  dry  coal,                    j 

17-5 

25.8 

18.1 

28.6 

42.8 

30-9 

30.2 

21.8 

Ratio  of  air  used  to  air  theoreti-  ) 
cally  required,                   J 

1.58 

2.40 

1.63 

2.61 

4.28 

2.82 

2.76 

I.98 

ANALYSIS  AND  CALCULATIONS. 

IX 

XI 

XII 

XIII 

XIV 

XVII 

XIX 

XX 

Per  cent  of  CO2, 

8.60 

16.50 

15.10 

17.94 

14.90 

II.  10 

"•53 

18.88 

Per  cent  of  CO, 

0.00 

0.21 

0.00 

1.02 

0.00 

o.oo 

0.00 

0.34 

Per  cent  of  O, 

14.40 

7.76 

8.80 

6-53 

6.60 

13.10 

13-03 

5-85 

Per  cent  of  N, 

77.00 

75-53 

76.10 

74-51 

78.50 

75.80 

75-44 

74-93 

Pounds  dry  air  per  pound  of  C, 

42.1 

21.2 

23-7 

18.2 

24.0 

32-7 

31.2 

18.3 

Pounds  dry  air  per  pound  of  coal, 

37-3 

18.8 

2I.O 

16.1 

21.3 

29.0 

27.6 

1  6.2 

Ditto,  per  pound  pure  dry  coal, 

38.6 

19.4 

21.7 

16.7 

22.0 

30.0 

28.6 

1  6.8 

Pounds  dry  furnace  gases  per  pound  \ 
pure  dry  coal,                     ) 

39-2 

2O.O 

22-3 

J7-3 

22.6 

30.6 

29.2 

17.4 

Ratio  of  air  used  to  air  theoreti-  \ 
cally  required,                   \ 

3-6 

1.81 

2.02 

1.56 

2.05 

2.80 

2.67 

1.56 

Accepting  12  pounds  of  air,  per  pound  of  fuel,  as  necessary  for  combustion, 
the  amount  required  where  100  per  cent  is  supplied  for  dilution,  as  in  the 
case  of  natural  draft  and  hand-firing,  will  be  24  pounds.  But  with  forced  draft 
the  quantity  of  air  required  for  dilution,  as  stated  by  Rankine,  ''is  certainly 
much  less  than  that  which  is  required  in  furnaces  with  chimney  draft ;  and  there 
is  reason  to  believe  that  on  an  average  it  may  be  estimated  at  about  one-half  of 
the  air  required  for  combustion."  That  is,  the  total  amount  would  be  18  pounds. 

This  applies  where  hand-firing  is  the  practice.  But  when,  through  the  action 
of  a  properly  applied  mechanical  stoker  supplied  with  air  under  pressure,  as  by 
means  of  a  fan,  the  bed  of  fuel  is  constantly  maintained  in  the  most  suitable 
condition  for  utilizing  the  air  supplied,  the  amount  required  for  dilution  is 
reduced  to  a  minimum.  This  is  particularly  true  when  the  stoker  grate  pro- 


MECHANICAL   DRAFT. 


27 


vides  special  advantages  for  the  equable  distribution  of  the  air.  Recent  tests, 
by  Mr.  J.  M.  Whitham,1  show,  not  only  the  decreased  air  supply  necessary  with 
a  good  mechanical  stoker,  but  also  the  reduction  in  the  amount  of  air  required 
per  pound  of  fuel  when  a  high  rate  of  combustion  is  maintained  by  the  use  of 
forced  draft.  With  a  combustion  of  twelve  pounds  of  buckwheat  coal  per 
square  foot  of  grate  per  hour,  the  air  was  found  to  be  85.6  per  cent  in  excess 
of  that  chemically  required;  while  with  a  rate  of  45.4  pounds  almost  perfect 
evaporative  efficiency  was  secured  when  there  was  an  actual  deficiency  of  11.2 
per  cent  in  the  air  supply  below  the  chemical  requirements.  Startling  as  this 
result  appears,  it  is  reported  by  an  able  expert  engineer.  It  certainly  points 
toward  the  possibilities  of  reduced  air  supply  with  mechanical  draft. 

"  In  almost  all  large  boiler  furnaces,"  as  stated  by  Prof.  H.  B.  Gale,2  "  a 
material  improvement  in  economy  may  be  made  by  cutting  down  the  grate  sur- 
face and  employing  forced  draft.  Theoretically,  12  pounds  of  air  are  sufficient 
to  completely  burn  a  pound  of  average  coal ;  but  in  practice,  with  large  grate 
surfaces  and  weak  draft,  between  20  and  30  pounds  are  required.  By  the 
employment  of  forced  draft  and  judicious  proportioning  of  the  furnace,  the 
quantity  of  air  may  be  reduced  easily  to  18  pounds,  with  the  result  of  a  white 
heat  in  the  furnace  and  better  combustion,  besides  the  saving  of  a  great  part  of 
the  expense  of  a  high  chimney." 

As  the  weight  of  dry  air  at  62°  is  0.0761  pounds  per  cubic  foot,  the  volumes 
corresponding  to  the  above  weights  are  as  indicated  in  Table  No.  17. 

Table  No.  17.  —  Amount  of  Air  Required  for  Combustion. 


Without  Dilution. 

With  50  per  cent 
Dilution. 

With  100  per  cent 
Dilution. 

Weight  of  air, 
Volume  of  air,  exact, 
Volume  of  air,  in  round  numbers, 

12  pounds. 
157.7  cu.  ft. 
150  cu.  ft. 

18  pounds. 
236.5  cu.  ft. 
225  cu.  ft. 

24  pounds. 
315.4  cu.  ft. 
300  cu.  ft. 

The  latter  figures,  given  in  round  numbers,  are  those  usually  employed. 

An  insufficient  supply  of  air  causes  imperfect  combustion  of  the  fuel,  which 
in  bituminous  coal  is  indicated  by  the  production  of  smoke,  and  in  coke  and 
anthracite  coal  by  the  discharge  of  carbonic  oxide  from  the  chimney.  An 


1  Experiments  with   Automatic  Mechanical  Stokers.      J.  M.  Whitham,  Trans.  Am.  Soc.  of 
Mech.  Engineers,  Vol.  XVII. 

2  Coal  as  a  Source  of  Power.     H.  B.  Gale.     A  paper  read  before  the  California  Electrical 

Society,  May  15,  1893. 


28  MECHANICAL   DRAFT. 

excess  of  air  causes  waste  of  heat  to  the  amount  corresponding  to  the  weight  of 
air  in  excess  of  that  which  is  necessary,  and  to  the  elevation  of  temperature  at 
which  it  is  discharged  from  the  chimney  above  that  of  the  external  air.  Ob- 
viously, the  maximum  efficiency  to  be  secured  in  the  process  of  combustion  is  to 
be  sought  between  these  two  extremes. 

Analysis  of  Flue  Gases.  —  It  is  a  comparatively  simple  matter,  by  means  of 
the  proper  apparatus,  to  determine  from  samples  the  relative  proportions  of  car- 
bonic oxide,  carbonic  acid  and  oxygen  in  the  gases  leaving  a  boiler  furnace. 
An  apparatus  of  this  character,  devised  by  Orsat,  consists  of  three  pipettes  in 
connection  with  a  graduated  burette  for  measuring  the  volumes  of  gas,  and  a 
pressure  bottle  to  control  the  movement  of  the  gases  undergoing  analysis.  The 
pipettes  contain  respectively  potassium  hydrate  for  the  absorption  of  carbonic 
acid,  an  alkaline  solution  of  potassium  pyrogallate  to  absorb  the  oxygen,  and 
cuprous  chloride  to  absorb  the  carbonic  oxide.  By  means  of  the  pressure  bottle 
the  sample  of  flue  gas  is  forced  through  the  pipettes  in  the  order  named,  and 
the  amount  absorbed  by  each  is  measured  by  means  of  the  burette.  The 
amounts,  by  volume,  thus  obtained  may  be  readily  transformed  into  amounts 
by  weight  by  multiplying  by  the  densities  of  the  various  gases.  Although  such 
an  analysis  does  not  directly  determine  the  amount  of  nitrogen  present  in  the 
flue  gases,  yet  its  actual  amount,  as  well  as  that  of  the  air  supply,  may  be  readily 
ascertained  by  calculation. 

To  illustrate  the  method  of  calculation,  take,  for  instance,  the  result  of  an 
analysis  showing  11.5  per  cent  of  carbonic  acid,  0.9  per  cent  of  carbonic 
oxide,  and  7.4  per  cent  of  free  oxygen,  all  by  volume.  Evidently,  the  nitrogen 
being  the  only  other  constituent  of  the  flue  gases  which  is  of  importance,  it  must 
be  present  in  sufficient  quantity  to  make  up  the  unit  volume  of  gas.  Its  volume 
will,  therefore,  be  — 

100  —  (i  1.5  -f-  0.9  -j-  7.4)  =  80.2  per  cent. 

In  the  calculation  of  the  weight  of  nitrogen  and  of  air  supply,  it  is  con- 
venient to  treat  the  percentages  by  volume  as  the  number  of  cubic  feet  of  the 
several  gases  in  100  cubic  feet  of  flue  gas.  Referring  to  Table  No.  14  for  the 
proper  volumes,  as  therein  given,  the  composition  of  the  flue  gas  by  weight 
appears  to  be  — 

Gas.  Volume.  Density.  Weight. 

Carbonic  acid,  11.5  0.12341  1.4192 

Carbonic  oxide,  0.9  0.07806  0.0703 

Oxygen,  7.4  0.08928  0.6607 

Nitrogen,  80.2  0.07837  6.2853 


MECHANICAL   DRAFT.  29 

As  the  atomic  weights  of  carbon  and  oxygen  are  respectively  12  and 
1 6,  it  is  evident,  as  is  shown  by  the  following  simple  calculation,  that  in  one 
pound  of  carbonic  acid  the  carbon  constitutes  — 

2  X 16  32         8 

12  -f-  (2  x  16)       44       ii 

of  the  weight,  the  remaining  T3T  being  oxygen.  In  a  similar  manner,  it  appears 
that  one  pound  of  carbonic  oxide  is  composed  of  — 

16        _  16 4 

12  -f-  16  ~  2~8  ~  7 

of  a  pound  of  oxygen  and  a  of 'a  pound  of  carbon.  Therefore,  the  weight  of 
oxygen  in  a  pound  of  the  above-stated  flue  gases  would  be  — 

In  the  carbonic  acid,  T8r  x  1.4192  =  .        1.0322  pounds. 

In  the  carbonic  oxide,  A  x  0.0703  =  .        0.0402        " 

Free  oxygen      .         .     •     .'•        .         .          .        0.6607        " 

Total  weight  of  oxygen  .          .        1.7331  pounds, 

and  the  weight  of  carbon  would  be  — 

In  the  carbonic  acid,  ^\  x  1-4192  =  .       0.3870  pounds. 

In  the  carbonic  oxide,  f  x  0.0703  =  .       0.0301        " 

Total  weight  of  carbon  .         .         .        0.4171  pounds. 

As  the  air  consists,  by  weight,  of  0.236  parts  of  oxygen,  the  above-estimated 
weight  of  oxygen  would  be  contained  in  — 

'''331  =  7.36  pounds  of  air, 
0.236 

and  the  supply  of  air  per  pound  of  carbon,  the  combustion  of  which  resulted 
in  flue  gases  having  the  composition  given  upon  the  preceding  page,  must, 
therefore,  have  been  — 

-^ —  =  17.61;  pounds. 
0.4171 

If  the  coal  from  the  combustion  of  which  these  gases  resulted  had  contained 
85  per  cent  of  carbon,  3.7  per  cent  of  hydrogen  and  2.4  per  cent  of  cocygen,  the 
air  supply  per  pound  of  coal  would  be  calculated  as  follows  :  The  supply  of 
air  per  pound  of  coal,  disregarding  the  oxygen  and  hydrogen  present  therein, 
would  be  — 

0.85  X  17-65  =  15.00  pounds. 


3o  MECHANICAL   DRAFT. 

But,  on  the  basis  already  established,  that  .the  oxygen  in  the  fuel  renders  inert 
one-eighth  of  its  weight  of  hydrogen,  and  the  remnant  is  available  for  combustion, 
there  would  be  added  to  the  air  per  pound  of  coal  — 

36  (0.037  —  °'°24)  =  0.468  pounds, 
8 

making  the  total  air  supply  per  pound  of  coal,  — 

15.00  -(-  0.468  =  15.468  pounds. 

Heat  of  Combustion. —  As  determined  by  the  most  recent  and  refined  calori- 
metric  tests,  the  heat  of  combustion,  as  measured  by  the  number  of  British 
thermal  units  that  are  given  out  upon  the  combustion  of  one  pound  of  a  given 
substance,  is  for  each  of  the  following  — 

Carbon  burned  to  CO2       ....  14,650  B.  T.  U. 

Carbon  burned  to  CO        ....  4,400       " 

Hydrogen  .         .         .  .         .  62,100       " 

Marsh  gas          ...         ...         .         .  23,513        " 

Olefiant  gas       .         ...         .         .         .  21,343       " 

Carbonic  oxide  burned  to  CO2  .         .         .  4>393        " 

The  great  loss  of  heat,  due  to  the  incomplete  combustion  of  carbon,  is  clearly 
presented  in  the  differences  between  the  total  heat  of  perfect  combustion  of 
carbon  to  CO2  (viz.,  14,650  B.  T.  U.),  and  that  of  carbon  to  CO  (viz.,  4,400 
B.  T.  U.)  ;  the  latter  being  the  product  of  incomplete  combustion  as  already 
stated  in  a  previous  section. 

One   pound  of   carbon,  when    imperfectly   burned,  produces   -         —  =  2  i/j 

pounds  of  carbonic  oxide.  If  this  quantity  of  gas  be  burned  to  form  carbonic 
acid,  the  total  amount  of  heat  given  out  will  be  14,650  —  4,400=10,250 
B.  T.  U.  ;  showing  that  ultimately  the  carbon  gives  out  its  full  heat  value,  no 
matter  what  the  order  of  formation  of  the  carbonic  acid  may  have  been,  whether 
by  direct  union  of  carbon  and  oxygen,  or  through  the  intermediate  agency  of 
the  carbonic  oxide.  As  the  10,250  B.  T.  U.  are  given  out  by  2^5  pounds  of 

carbonic  oxide,  its  heat  value  per  pound  is,  evidently,  I0'^5°  =  4,393  B.  T.  U. 

2^3 

In  calculating  the  heat  of  combustion  of  a  fuel,  it  is  customary  to  disregard, 
as  already  explained,  that  portion  of  the  hydrogen  for  which  there  exists  in  the 
fuel  a  sufficient  amount  of  oxygen  to  form  water.  The  remainder  of  the  carbon 
and  hydrogen  thus  becomes  available  for  producing  heat,  and  may  be  intro- 
duced in  an  approximate  formula,  based  upon  that  for  estimating  the  air 


MECHANICAL    DRAFT. 


31 


required   for  combustion.     Disregarding   the  effect   of  inherent  nitrogen    and 
sulphur,  this  formula  may  be  thus  expressed  :  — 

Heat,  in  B.  T.  U.,  =  14,650  C  —  62,100  (H  —  —), 

8 

in  which  the  weights  of  carbon,  hydrogen  and  oxygen  in  one  pound  of  fuel  are 
respectively  represented  by  their  symbols,  C,  H  and  O. 

This  formula,  applied  to  the  determination  of  the  heat  of  combustion  of  the 
Maryland  semi-bituminous  coal  in  Table  N/x  13,  appears  as  follows:  — 


Heat,  in  B.  T.  U.,  =  14,650  x  0.80  +  62,100  (0.05  — 


14,615. 


Theoretically,  the  total  calorific  value,  as  determined  by  the  calorimeter  and 
as  calculated  from  analysis,  should  agree.  But,  on  the  one  hand,  there  is 
opportunity  for  error  or  imperfection  on  the  part  of  the  calorimeter  ;  while  on 
the  other,  the  formula  employed  for  calculation  from  the  analysis  may  fail  to 
make  due  allowance  for  heat  lost  in  dissociation,  or  may  not  properly  recog- 
nize the  influence  of  minor  constituents.  In  both  cases  there  is  great  dif- 
ficulty in  obtaining"  similar  samples.  This  accounts  for  differences  which 
frequently  exist  in  reported  results.  Thus,  the  calorimetric  tests  of  Scheurer- 
Kestner  were  about  ten  per  cent,  on  an  average,  higher  than  the  analyses  ;  while 
results  reported  by  Mr.  F.  W.  Dean1  show  the  calorific  value,  as  determined  by 
calorimeter,  to  be  about  six  per  cent  less  than  that  calculated  from  the  test. 
A  comparison  of  the  results  obtained  by  these  two  methods  of  determination  is 
presented  in  Table  No.  18,  from  the  tests  of  Mahler  on  various  American  and 
foreign  coals. 

Table  No.   18.  —  Heat  of  Combustion  of  Fuels. 


Analysis. 

•Sfc-S-S 

| 

3    -g 

•I-d 

Kind  of  Fuel. 

iii 

i 

ift 

ffi 

Carbon. 

Hydro- 

Oxygen 

and   Ni- 

Hydro- 
scopic 

Ash. 

gen. 

trogen. 

Water. 

*-'•£  K  rt 

•lo03 

"rt 

lo" 

0.          f 

U 

U      £. 

o 

Anthracite,  from  Penna., 

86.456 

1-995 

2.199 

'  3-45° 

5.900 

3-0° 

13,47! 

I4,86l 

15,210 

Semi-anth.,  from  Commentry, 

84.928 

2.892 

5-005 

i-775 

5.400 

3-19 

14,130 

15,221 

15,048 

Semi-bituminous,fromAniche 

85.937 

4.198 

5.240 

0.625 

4.OOO 

"•93 

15,167 

15,901 

15,638 

Bituminous,  from  Anzin, 

83.754 

4.385 

5-76I 

I.IOO 

5.000 

21.51 

14,492 

1  5.433 

15,640 

Wigan  cannel  coal, 

78.382 

5.060 

5.058 

0.600 

10.900 

31.64 

13,970 

15,682 

16,220 

Lignite,  from  Styria, 

65-455 

4.782 

24-303 

0.710 

4-75° 

50-34 

I3."1 

",963 

11,898 

Coke,  Penna.  anthracite, 

91.036 

0.685 

2.146 

o-233 

'  5-900 

68.93 

13.550 

14,465 

14,540 

Transactions  Am.  Soc.  Mech.  Engineers,  Vol.  XVII.,  p.  285. 


MECHANICAL    DRAFT. 


In  boiler  practice,  owing  to  the  opportunities  for  loss  of  heat  through  radia- 
tion, heat  carried  off  by  flue  gases,  incomplete  combustion,  etc.,  the  maximum 
efficiency  attainable  with  the  best  possible  boiler  and  warm-blast  or  feed-water 
heating  apparatus  appears  to  be  about  90  per  cent.  Under  ordinary  conditions 
with  good  coal,  the  efficiency  may  be  assumed  to  average  from  60  to  70  per  cent, 
and  with  poor  coals  from  50  to  60  per  cent.  It  is,  therefore,  customary,  for 
rough  figuring,  to  consider  the  available  heat  of  combustion  per  pound  of  fuel 
to  be  ordinarily  from  10,000  to  12,000  B.  T.  U. 

The  total  heat  of  various  fuels  will  be  shown  in  succeeding  tables. 

Ideal  Temperature  of  Combustion.  —  From  the  known  total  and  specific  heats 
of  combustibles  may  be  calculated  the  temperature  which  would  result  from 
their  combustion  if  all  possible  losses  were  prevented.  In  ordinary  practice 
those  losses  must  occur  and  the  efficiency  of  fuels  be  reduced  thereby.  It  is, 
therefore,  impossible  to  attain  in  practice  the  full  ideal  temperature.  The  gen- 
eral properties  of  the  substances  entering  into  a  discussion  of  the  combustion 
of  fuels  are  given  in  Table  No.  19. 

Table  No.   19.  —  Properties  of  Substances   Concerned  in  Combustion. 


Substance. 

Symbol. 

Atomic 
or  Molecular 
Weight. 

Specific 
Volume. 

Specific  Heat 
in  a  Gaseous 
Condition. 

Density 
or  Weight 
per  Cubic  Foot. 
Pounds. 

Hydrogen, 

H 

, 

178.881 

3409 

0.00559 

Carbon, 

C 

12 







Nitrogen, 

N 

14 

12.7561 

0.2438 

0.07837 

Oxygen, 

0 

16 

11.2070 

0.2175 

0.08928 

Carbonic  oxide, 

CO 

12  +  16 

I2.8l 

0.2450 

0.07806 

Carbonic  acid, 

CO2 

12  +  2   X    l6 

8.10324 

0.2169 

0.12341 

Water, 

H2O 

2+16 



0.4805 



Air, 

12.3909 

0.2375                0.08071 

Ash, 

O.2 

It  has  already  been  shown  that  one  pound  of  carbon,  burned  to  carbonic  acid, 
requires  23/3  pounds  of  oxygen.  Hence  the  total  product  of  combustion  of  one 
pound  of  carbon  =  3^  pounds,  as  is  also  evident  by  the  following  calculation 
based  upon  the  atomic  weights  :  — 


I2-t-(2    X 


=  Z2A  pounds. 


It  has  further  been  shown  that  a  total  of  11.3  pounds  of  air  is  required  to 
furnish  2^3  pounds  of  oxygen.     Therefore,  the  total  weight  of  the  products  or 


MECHANICAL   DRAFT.  33 

results  of  combustion  must  be  12.3  pounds,  and  the  weight  of  the  nitrogen  alone 
12.3  — 3^3  =  8.63  pounds.  As  the  specific  heat  of  a  substance  is  a  measure  of 
the  number  of  thermal  units  necessary  to  raise  its  temperature  through  one 
degree,  the  total  number  of  units  required  to  raise  through  one  degree  the  pro- 
ducts of  combustion  of  one  pound  of  carbon,  with  the  associated  nitrogen,  may 
be  determined  thus  :  — 

Weight.       Specific  Heat.          B.  T.  U. 

Carbonic  acid  .         .         .         3^3    x    0.2169  =   °-7953 

Nitrogen         ...         .         .         .    '     8.63  x    0.2438  =   2.1040 

B.  T.  U.  per  degree        ...        .         .         .         .      2.8993 

As  one  pound  of  carbon  in  the  process  of  burning  gives  out  14,650  B.  T.  U., 
and  as  it  requires  2.8993  B.  T.  U.  to  raise  through  one  degree  the  products  of 
combustion,  including  the  accompanying  nitrogen,  the  ideal  temperature  result- 
ing from  the  combustion  of  one  pound  of  carbon  must  be  14,650-^2.8993  = 
5,053°.  In  the  same  manner  the  ideal  temperature  of  combustion  of  hydrogen 
may  be  calculated,  and  as  it  makes  no  difference  in  the  temperature  whether 
the  oxygen  required  for  this  union  is  derived  from  the  original  constituents  of 
the  fuel  or  from  the  atmosphere,  the  entire  amount  of  hydrogen  in  the  fuel  is 
taken  into  account. 

For  the  purpose  of  illustrating  the  process  of  calculation,  the  Maryland  semi- 
bituminous  coal  in  Table  No.  13  may  be  again  considered.  The  important 
constituents  of  this  coal,  expressed  in  per  cent  of  one  pound  of  coal,  are  — 

Carbon 80.0  per  cent. 

Hydrogen  .          .         .         .         .  .  5.0         " 

Oxygen 2.7 

Nitrogen  .         .         .         .         .         .         .  i.i         " 

For  simplicity  the  ash  and  sulphur  may  be  disregarded,  and  also  the  latent 
heat  of  the  steam  formed  by  the  combination  of  hydrogen  and  oxygen.  The 
heat  of  combustion  of  this  coal  has  already  been  calculated  as  14,615  B.  T.  U. 
By  the  process  explained  in  the  section  on  "  Air  Required  for  Combustion,"  it 
has  also  been  shown  that,  for  the  total  combustion  of  the  carbon  and  hydro- 
gen contained  in  one  pound  of  this  coal,  there  are  required  10.62  pounds  of 
air,  of  which  2.51  pounds  will  be  oxygen  and  8.11  pounds  will  be  nitrogen. 
This  amount  of  nitrogen,  added  to  that  already  in  the  coal,  makes  the  total 
8. 1 1  -|-  1. 1  =9.21  pounds. 

The  total  amount  of  carbonic  acid  produced  by  the  union  of  oxygen  with 
0.8  pounds  of  carbon  is  ^2A  x  °-8  =  2-933  Pounds'  and  as  tne  total  products  of 


34  MECHANICAL    DRAFT. 

combustion  of  one  pound  of  hydrogen  are  —       —  =9   pQunds,   the    weight  of 

the  products  of  combustion  of  the  hydrogen  in  the  coal  will  be  9  X  0.05  =  0.45 
pounds.  Hence,  the  thermal  units  required  to  raise  each  of  these  combustibles 
through  one  degree  are  — 

Weight.        Specific  Heat.          B.  T.  U. 

Carbonic  acid        .         .          •         2-933    x    0.2169   =  0.6362 
Water   .....  0.45    x    0.4805    =  0.2162 

Nitrogen        ....  9.21    x    0.2438   =  2.2454 

Total  B.  T.  U 3-0978 

The  ideal  temperature  of  combustion,  therefore,  appears  to  be  — 
14,615  -^  3. 0978  =  4,718°. 

If,  for  the  purposes  of  dilution,  there  had  been  provided  50  per  cent  of  air  in 
excess  of  that  theoretically  required  for  complete  combustion,  the  amount  of 
heat  necessary  to  raise  the  temperature  of  the  products  of  combustion  through 
one  degree  would  have  been  increased,  and  the  final  temperature  reduced,  as 
is  evident  from  the  following  :  — 

Weight.  Specific  Heat.      B.  T.  U. 

50  per  cent  air  for  dilution,    -     —  =  5.31  x  0.2375  =  1.2611 

Products  without  dilution  as  above  =  3.0978 

Total  B.  T.  U.  .         .         .         .  4-3589 

and  14,615-4-4.3589=3,353°.  The  cooling  effect  of  the  air,  which  is  abso- 
lutely necessary  for  dilution,  is  thus  made  evident  by  a  decrease  of  4,718  — 
3,353  =  I,365°  when  it  is  only  50  per  cent  in  excess. 

While  the  temperature  of  combustion  of  a  complex  fuel  may  be  calculated 
with  much  greater  refinement  by  taking  into  account  all  of  the  minor  constitu- 
ents, the  results  thus  obtained  are  practically  of  but  little  more  value  than  those 
derived  from  this  approximate  method ;  for  local  conditions  in  boiler  practice 
always  have  considerable  effect  in  reducing  the  actual  temperature  to  some- 
what below  the  ideal.  Mr.  J.  C.  Hoadley,1  in  carefully  conducted  tests  with  a 
water-platinum  calorimeter,  found  in  the  heart  of  the  fire  under  an  ordinary 
boiler  a  temperature  of  2,426°,  the  coal  consisting  of  82  per  cent  of  carbon, 
and  the  supply  of  air  being  21.4  pounds  per  pound  of  coal.  Immediately  above 
the  fire,  and  at  the  bridge  wall,  the  temperature  rapidly  decreased  through  losses 


Warm-Blast  Steam-Boiler  Furnace.     J.  C.  Hoadley.     New  York,  1886. 


MECHANICAL    DRAFT. 


35 


by  radiation  and  conduction  to  the  walls  and  the  water  in  the  boiler,  so  that  the 
corresponding  temperature  at  the  bridge  wall  was  only  1,341°. 

The  ideal  temperature  of  combustion  of  the  Maryland  semi-bituminous  coal, 
with  different  degrees  of  dilution,  as  determined  by  calculation  in  the  manner 
already  indicated,  is  presented  in  Table  No.  20. 

Table  No.  20.  —  Ideal  Temperatures  of  Combustion  with  Different  Degrees  of  Dilution. 


Percentage  of  Dilution. 

Ideal  Temperature. 

Loss  of  Temperature  due  to 
Dilution. 

0 

4,7180 

5° 
100 

3-353 
2,600 

I,3650 
2,1  18 

'5° 

2,124 

2,594 

These  figures  indicate  only  the  increments  of  temperature  under  the  given 
conditions ;  hence,  to  obtain  the  actual  thermometric  temperature,  they  must  be 
increased  by  the  initial  temperature  of  the  air.  Thus,  if  the  air  is  supplied  at 
62°,  the  ideal  temperature,  with  100  per  cent  dilution,  would  be  2,600  -{-62  = 
2,662°,  while  if  the  air  had  been  previously  heated  by  special  means  to  300°,  it 
would  be  2,600  -j-  300  =  2,900°. 


CHAPTER    IV. 
FUELS. 

Definition.  —  Fuels  maybe  defined  as  those  substances  which,  by  means  of 
atmospheric  air,  can  be  economically  burned  to  generate  heat.  The  principal 
constituent  of  all  is  carbon,  with  which  hydrogen  is  usually  associated.  They 
may  be  broadly  classified  as  natural  and  artificial. 

Natural  Fuels.  —  Natural  fuels  are  such  forms  of  carbon  and  its  compounds 
with  hydrogen  as  occur  distributed  in  nature,  either  as  products  of  existing 
organic  life,  or  as  the  fossilized  remains  of  a  prehistoric  growth.  Under  this 
heading  are  included  the  varieties  of  wood,  coal,  mineral  oil  and  natural  gas. 
The  solid  fuels  may  be  classified  as  follows :  — 

WOOD. 
PEAT. 

C  Lignite. 

f  Non-caking,  rich  in  oxygen. 
COAL,      <j  Bituminous,      •<  Caking. 

(  Non-caking,  rich  in  carbon. 
[_  Anthracite. 

Artificial  Fuels.  —  Artificial  fuels  comprise  those  forms  of  carbon  or  its  com- 
pounds with  hydrogen  which  owe  their  origin  to  some  process  of  manufacture, 
but  are  not  commonly  found  distributed  in  nature.  These  are  generally  obtained 
from  natural  fuels  by  some  special  process;  as,  for  instance,  charcoal  from 
wood,  coke  and  volatile  hydro-carbons  from  coal.  Artificial  fuels  include  the 
various  attempts  to  cement  together,  in  the  form  of  blocks  or  briquettes,  such 
combustible  refuse  as  is  too  small  to  be  otherwise  profitably  consumed. 

The  products  of  carbonization  may  be  classified  as  follows  :  — 


f 


Solid, 


Wood  —  Charcoal. 
Peat  —  Charcoal. 


Coke. 
PRODUCTS  OF 

CARBONIZATION, 

Carbonic  oxide. 


Volatile,       )  Hydrogen. 

(  Hydro-carbons. 


MECHANICAL   DRAFT. 


37 


Wood.  —  Although  the  term  "wood"  broadly  includes  all  substances  of  vege- 
table fibre  which  have  not  undergone  geological  changes,  it  applies  directly  to 
the  fairly  compact  substance  which  constitutes  tree  trunks  and  branches.  With 
reference  to  its  heating  power,  wood  under  this  definition  may  be  classed  as  hard 
and  soft.  Hard  woods  include  the  oak,  hickory,  maple,  beech  and  walnut;  and 
soft  woods,  the  pine,  elm,  birch,  chestnut,  poplar  and  willow.  When  freshly  cut, 
wood  contains  nearly  fifty  per  cent  of  moisture,  which  seriously  reduces  its  cal- 
orific value.  Through  the  process  of  air  or  kiln  drying,  the  amount  of  moisture 
may  be  brought  down  to  from  10  to  20  per  cent. 

The  approximate  weight  of  one  cord  of  thoroughly  air-dried  wood,  and  its 
calorific  value  relatively  to  soft  coal,  are  given  in  Table  No.  21,  for  the  kinds 
specified. 

Table  No.  21.  — Weight  and  Calorific  Value  of  Wood. 


KIND  OF  WOOD. 

Weight. 

Weight  of  Coal  of 
Equivalent  Calorific  Value. 

Hickory  or  hard  maple, 

4,500  pounds. 

i,  800  pounds. 

White  oak, 

3.850        " 

1,540        " 

Beech,  red  and  black  oak, 

3,250 

1,300        " 

Poplar,  chestnut  and  elm, 

2,350        " 

940 

Average  pine, 

2,000        " 

800        " 

The  above  indicates  that  about  2^  pounds  of  dry  wood  are  equal  to  a  pound 
of  average  soft  coal,  and  that  the  calorific  value  of  the  same  weight  of  various 
woods  is  substantially  the  same.  The  average  chemical  composition  of  the 
ordinary  kinds  of  wood,  when  perfectly  dry,  is  shown  by  Table  No.  22  to  be 
substantially  the  same. 

Table  No.  22.  —  Composition  of  Wood. 


Carbon. 

Hydrogen. 

Oxygen. 

Nitrogen.     ! 

Ash. 

Beech, 

49-36 

6.01 

42.69 

0.91 

1.  06 

Oak, 

49.64 

5-92 

41.16 

1.29 

1.97 

Birch, 

50.20 

6.20 

41.62 

I.IS 

0.8  1 

Poplar, 

49-37 

6.21 

41.60 

0.96 

1.86 

Willow, 

49.96 

5.96 

39-56 

0.96 

3-37 

Averages, 

49.70 

6.06 

41.30 

..»5 

i.  80 

MECHANICAL   DRAFT. 


Calculated  by  the  usual  formula  as  already  presented,  the  heat  value  of  wood 
of  the  average  composition,  shown  in  Table  No.  22,  would  be  7,838  B.  T.U. 
If  this  wood  is  considered  to  be  in  the  ordinary  condition  of  air-dried  stock, 
containing  about  20  per  cent  of  moisture,  then  its  heating  value  per  pound 
would  be  only  three-quarters  as  much,  or  5,879  B.  T.  U. 

Straw  and  Tan.  —  Evidently  straw  can  be  economically  employed  as  a  fuel 
only  where  the  supply  is  directly  at  hand  and  the  cost  of  other  fuel  is  excessive. 
Analyses  of  air-dried  straw  have  shown  it  to  be  of  the  composition  indicated  in 
Table  No.  23. 

Table  No.  23.  —  Composition  of  Straw. 


CONSTITUENTS. 

Wheat  Straw.              Barley  Straw. 

Mean. 

Carbon, 

35.86                           36.27 

36.0 

Hydrogen, 

5-oi 

5-°7 

5-° 

Oxygen, 

37.68 

38.26 

38.0 

Nitrogen, 

0-45 

0.40 

0.50 

Ash, 

5-00 

4.50 

4-75 

Water, 

1  6.00 

'5-50  . 

I5-75 

Straw  of  the  mean  composition  given  above  has  a  calorific  value,  deducting 
the  heat  lost  in  evaporating  its  constituent  water,  of  5,155  B.  T.  U.  It  weighs, 
when  pressed,  6  to  8  pounds  per  cubic  foot.  Oak  bark,  after  having  served  its 
purpose  as  a  tanning  agent,  thereby  becoming  spent  tan  and  consisting  only 
of  the  fibrous  portion  of  the  bark,  is  used  as  a  fuel,  but  only  under  the  economi- 
cal conditions  which  hold  in  the  use  of  straw  as  a  fuel.  That  is  when  the 
tan  is  readily  accessible  and  its  total  cost  when  placed  in  the  furnace  is  less, 
for  a  given  result,  than  that  of  other  available  fuels.  In  the  process  of 
tanning  the  bark  loses  about  20  per  cent  of  its  weight.  Perfectly  dry  tan, 
containing  15  per  cent  of  ash,  has  a  heating  power  of  6,100  B.  T.  U. ;  while  tan 
containing  30  per  cent  of  water  —  its  usual  condition  of  dryness  —  has  a  cal- 
orific value  of  only  4,284  B.  T.  U.  The  weight  of  water  evaporated  at  212°  by 
one  pound  of  tan  under  these  two  conditions  of  dryness  is  as  follows  :  — 

Perfectly  Dry.  With  30  Per  Cent  of  Moisture. 

Water  supplied  at  62°  .     5.46  pounds.  3.84  pounds. 

Water  supplied  at  212°        .     6.31  pounds.  4.44  pounds. 

The  conditions  of  success  in  burning  tan,  as  is  the  case  with  all  wet  fuel, 
consist  in  completely  surrounding  it  with  heated  surfaces  and  burning  fuel  so 
that  it  may  be  rapidly  dried,  and  then  so  arranging  the  apparatus  that  thorough 
combustion  may  be  secured. 


MECHANICAL   DRAFT. 


39 


,  —  The  term  "bagasse,"  or  megass,  is  generally  understood  to  apply 
to  that  portion  of  the  sugar  cane  that  is  left  after  extracting  the  juice.  As  the 
methods  of  extraction  give  results  varying  all  the  way  from  40  per  cent  to  80 
per  cent,  it  is  evident  that  it  includes  substances  differing  greatly  in  composi- 
tion. In  its  broadest  sense  it  may,  therefore,  be  taken  as  meaning  the  refuse 
discharged  from  the  cane  mill  or  diffusion  process,  whether  it  comes  from  a  mill 
giving  40  per  cent  extraction  and  leaving  70  per  cent  of  moisture,  or  whether  it 
be  the  air-dried  bagasse  of  the  tropics  with  only  10  per  cent  of  moisture. 

Mill  bagasse  is  the  refuse  left  after  the  juice  has  been  extracted  by  means  of 
the  mill  rolls.  Diffusion  bagasse  is  the  material  remaining  after  a  series  of 
soaking  processes  for  which  it  has  been  chopped  into  small  pieces,  and  whereby 
the  saccharine  matter  has  diffused  itself  throughout  the  mass  of  water  in  which 
the  cane  has  been  placed.  The  original  cane,  and  likewise  the  bagasse,  consist 
of  woody  fibre,  water  and  combustible  salts ;  but,  naturally,  the  squeezing  pro- 
cess reduces  the  percentage  of  liquid  matter,  and  proportionately  increases  the 
relative  amount  of  fibrous  material  in  the  bagasse.  Upon  the  fibre,  which  is 
principally  carbon,  the  value  of  bagasse  as  a  fuel  largely  depends.  In  tropical 
canes  it  constitutes  in  round  numbers  about  12  per  cent  of  the  original  cane, 
while  in  Louisiana  cane  10  per  cent  is  a  fair  average.  Tropical  cane  and  the 
bagasse  therefrom  have  the  composition  given  in  Table  No.  24. 

Table  No.  24. —  Composition  of  Tropical  Cane  and  Bagasse. 


BAGASSE. 

66  per  cent 
Extraction. 

70  per  cent 
Extraction. 

72  per  cent 
Kxtraction. 

Woody  fibre, 

12.5 

37 

40 

45 

Water, 

73-4 

53                     5° 

46 

Combustible  salts, 

14.1 

10 

10 

9 

The  proportional  composition  of   Louisiana  bagasse  is  clearly  shown,  for  dif- 
ferent degrees  of  extraction,  in  Table  No.  25. 

Table  No.  25.  —  Composition  of  Dry  Louisiana  Bagasse. 


CONSTITUENTS. 


Percentage. 


Volatile  matter, 
Fixed  carbon, 
Ash, 


Si-37 
14.26 
4-6 


MECHANICAL    DRAFT. 


Carefully  conducted  calorimetric  tests  of  bagasse,  when  under  different  con- 
ditions, by  Dr.  W.  O.  Atwater,  give  the  heat  values  which  are  indicated  in  Table 
No.  26. 

Table  No.  26.  —  Calorimetric  Tests  of  Bagasse. 


DESCRIPTION. 

Per  cent 
Moisture  in 
Sample. 

B.  T.  U. 
per  Pound  as 
received. 

B.  T.  U.  per 
Ib.  Dry  Matter 
from  Preceding 
Column. 

B.  T.  U.  per 
Ib.  Dry  Matter 
by  Actual  Test. 

Purple   cane    exhaust   chips, 
from  diffusion  battery, 

direct   ( 
$ 

90.36                   799 

8,288 

8,320 

Striped  cane  exhaust   chips, 
from  diffusion  battery, 

direct   ) 

90-54                         873 

9,229 

8,289 

Purple  cane   exhaust  chips, 
through  laboratory  mill, 

passed   ) 

73-34 

I,966 

7,373 

8.3C9 

Striped  cane  exhaust  chips, 
through  laboratory  mill, 

passed   ) 

69.62 

2,547 

8,384 

8,384 

Averages, 

8,319 

8,325 

This  table  serves  to  show  the  serious  effect  of  contained  water  upon  the  heat 
value  of  the  bagasse  of  different  extractions.  This  effect  is  to  be  expected, 
for  all  of  the  water  in  the  bagasse  when  employed  as  fuel  must  be  vaporized 
before  the  combustible  matter  can  be  consumed,  and  in  the  process  of  vaporiza- 
tion an  enormous  amount  of  heat  is  rendered  latent,  and  thus  lost  to  the  fur- 
nace so  far  as  heating  effect  is  concerned. 

The  only  method  available  in  estimating  the  fuel  value  of  the  different  extrac- 
tions of  mill  bagasse  is  that  based  upon  the  assumption  that  bagasse  consists 
of  two  substances, —  fibre  and  juice, —  and  that  this  juice  has  the  same  compo- 
sition as  that  which  has  already  been,  extracted.  To  obtain  a  heat  value  for 
juice,  it  must  be  divided  into  sugar  and  molasses.  Thus,  for  instance,  an  aver- 
age cane  consisting  of  — 

Fibre          .... 
r  Sucrose 

TuiceJ  Glucose 

|  Solids  —  not  sugar, 

I  Water 


10  per  cent, 
12        " 

2  " 

I  " 

75 


ioo  per  cent, 

will,  upon  passing  through  a  mill  giving  an  extraction  of  75  per  cent,  be  reduced 
to  bagasse,  the  weight  of  which  will  be  only  25  per  cent  of  the  original  cane  of 
which  it  formed  a  part. 

The  proportional  composition  in  per  cent  of  the  weight  of  the  original  cane 
and  of  the  resulting  bagasse  will  then  be  as  presented  in  Table  No.  27. 


MECHANICAL    DRAFT. 


Table  No.  27.  —  Composition  of  Mill  Bagasse. 


CONSTITUENTS. 

In  per  cent  of 
Original  Cane. 

In  per  cent  of  the 
Resulting  Bagasse. 

Water, 

12.75 

5' 

Fibre, 

10.0 

40 

Sugar, 

'•5 

6 

Molasses  (dry  matter  only), 

0-75 

3 

' 

25.00 

JOO 

Calorimetric  tests  of  molasses,  sugar  and  fibre  indicate  the  following  values:  — 


Molasses 

Sugar 

Fibre 


6,956  B.T.U. 

7,223        « 
8,325        " 


In  Table  No.  28  the  total  heat  values  are  based  upon  those  of  the  constituents 
given  above,  and  a  fair  sample  of  Pennsylvania  coal  having  a  heat  value  of 
14,000  B.T.  U.  is  taken  as  the  basis  of  comparison  with  coal. 

Table  No.  28.  —  Value   of  One   Pound  of   Mill   Bagasse   at   Different   Extractions  upon 
Cane  of  10  per  cent  Fibre  and  Juice  of  15  per  cent  Total  Solids. 


Per  cent 

|. 

FIBRE. 

SUGAR. 

MOLASSES. 

!* 

ifc 

i- 

""rt^eS 

11 

1 

"5 

Extraction 
on  Weight 
of  Cane. 

|l 

"  nf 

Fuel  Value. 
B.  T.  U. 

If 

>H 
ga 

if 
fl 

Fuel  Value. 
B.  T.  U. 

Total  Heat 
oped.  B.  : 

Heat  requii 
evaporate  the 
present.  B. 

K 

Iflj 

I1 

Temperature  < 
Fahr. 

| 

90 

0.00 

loo.oo    8,325 

— 

— 

— 

— 

8,325 

— 

8,325 

1.68 

119 

2,4650 

85 

28.33 

66.67 

5-550 

3-33 

240 

1.67 

116 

5,900 

339 

5,56i 

2.52 

119 

2,236 

80 

42.50 

50.00 

4,162 

5.00 

361 

2.50 

J74 

4,697 

5°9 

4,188 

3-34 

1  20 

2,023 

75 

51.00 

40.00   3,330 

6.00 

433 

3-oo 

209 

3-972 

6n 

3,36i 

4.17 

120 

1,862 

70 
65 

56.67 

60.71 

33-33 

28.57 

!  2,775 
2,378 

6.67 
7-15 

482 

3-33 

3-57 

232 
248 

3,489 

679 

727 

2,810 
2,415 

4.98 

I  2O 
121 

1,732 
1,612 

60 

63-75 

25.00 

2,081 

7-50 

541 

3-75 

261 

2^883 

764 

2,119 

6.6  1 

121 

1,513 

55 

66.12 

22.22 

1,850 

7.78 

562 

3.88 

270 

2,682 

792 

1,890 

7.40 

121 

1,427 

50 

68.00 

2O.OO 

1,665 

8.00 

578 

4.00 

278 

2,521 

815 

1,706 

•   8.21 

122 

i,35° 

45 

69-55 

18.18 

1,513 

8.18 

591 

4.09 

284 

2,388 

833 

',555 

9.00 

122       1,284 

40 

70.83 

16.67 

1,388 

8-33 

601 

4.17 

290 

2,279 

849 

i,43° 

9-79 

I23       1,222 

25 

73-67 

13.33     MIO 

8.67 

626 

4-33 

301 

2,037 

883 

1,154 

12.13 

124 

1,077 

15 

75.00 

11-77  ;        980 

8.82 

637 

4.41 

307 

1,924 

899 

1,025 

13.66 

124       1,002 

0 

76.50 

10.00        832 

9.00 

650 

4.50 

313 

i,795 

916 

879 

15-93 

1  26           906 

MECHANICAL    DRAFT. 


In   somewhat  abbreviated  form  the  fuel   values   of  one   pound  of  diffusion 
bagasse,  at  various  degrees  of  moisture,  are  given  in  Table  No.  29. 

Table  No.  29. — Fuel  Values  of  One  Pound  of  Diffusion  Bagasse  at  Various  Degrees 

of  Moisture. 


Moisture  in  Bagasse. 

Heat  developed  per 
Pound  of  Bagasse. 

Heat  Available  per 
Pound  of  Bagasse. 

Number  of  Pounds 
of  Baeasse  Equivalent 

Estimated 
Temperature  of  Fire. 

Per  cent. 

B.  T.  U. 

B.  T.  U. 

to~i  Ib.  of  Coal. 

Fahr. 

0 

3-325 

8,325 

1.68 

2,4650 

20 

6,660 

6,420 

2.18 

2,294 

30 

5,827 

5,468 

2.56 

2,186 

40 

4,995                         4-51  6 

3.10 

2,049 

5° 

4,162 

3,563 

3-93 

1,870 

60 

3,330 

2,611 

5-4i 

I,627 

70 

2,497                          1,658 

8-44 

I,28l 

75 

2,081                          1,183 

11.90 

1,045 

Peat.  - —  Intermediate  between  wood  and  coal  may  be  placed  peat,  which  is 
the  result  of  one  of  the  most  important  geological  changes  now  in  progress.  In 
certain  swampy  regions  in  the  temperate  latitudes  there  occur  immense  quanti- 
ties of  semi-aquatic  plants,  which,  under  special  conditions  of  heat  and  moisture, 
are  undergoing  a  curious  chemical  transformation,  whereby  the  oxygen  of  the 
plant  is  eliminated,  leaving  behind  as  peat  a  spongy  carbonaceous  residue.  This 
is  found  in  beds  varying  from  i  or  2  to  40  feet  in  depth.  That  near  the  surface, 
which  is  in  a  less  advanced  state  of  decomposition,  is  light,  spongy  and  fibrous, 
of  yellow  or  light  reddish-brown  color ;  lower  down  it  is  more  compact,  and 
darker  in  color;  while  in  the  lowest  strata  the  color  is  almost  black,  and  the  peat 
is  pitchy  and  unctuous,  with  scarcely  any  evidence  of  the  fibrous  texture  which 
exists  in  the  higher  strata  and  in  the  original  vegetable  matter  from  which  it  was 
formed. 

In  its  natural  condition,  peat  generally  contains  from  75  to  80  per  cent  of  its 
entire  weight  of  water,  occasionally  amounting  to  85  or  even  90  per  cent.  Evi- 
dently it  is  thus  totally  unfitted  for  use  as  a  fuel  until  it  has  been  dried.  By  the 
process  of  drying  it  shrinks  very  decidedly,  its  specific  gravity,  when  dry,  vary- 
ing from  0.22  or  0.34  for  the  surface  peat,  which  is  light  and  porous,  to  1.06  for 
the  lowest  peat,  which  is  very  dense.  Owing  to  the  abundance  of  other  fuels, 
peat  has  been  but  little  used  in  this  country ;  but  in  Ireland,  Germany  and 
Sweden  it  has  already  found  an  extensive  field,  not  only  in  domestic  but  in 
metallurgical  operations. 


MECHANICAL   DRAFT. 


43 


The  composition  of  ordinary  Irish  peats,  both  exclusive  and  inclusive  of  the 
moisture,  which  they  always  contain  in  their  natural  condition,  is  displayed  here- 
with in  Table  No.  30. 

Table  No.  30.  —  Composition  of  Irish  Peats. 

EXCLUSIVE  OF  MOISTURE. 


DESCRIPTION. 

Moisture. 

Carbon. 

Hydrogen    Oxygen.     Nitrogen 

j  Sulphur. 

A,H.      j 

Coke. 

Good  air-dried, 



59-7 

6.0                   31.9 

2-4 



Poor  air-dried, 



59-6 

4-3 

29.8 



6-3 



Dense,  from  Galway, 



59-5 

7-2 

24-8     |       2.3 

08 

5-4 

44-3 

Averages, 

;  59-6 

5.8                   29.6 

°3 

47 



INCLUSIVE  OF  MOISTURE. 


i 

Good  air-dried, 

24.2 

45-3 

4.6 

24.1 

— 

1.8 



Poor  air-dried, 

29.4 

42.1 

3-i 

21.0 



4-4 

Dense,  from  Galway, 

29-3 

42.0 

5-i 

17-5     1       i-7 

0.6 

3"8 

3i-3 

Averages, 

27.8 

43-i 

4-3 

21.4 

0.2 

3-3      ' 

The  average  composition  of  Irish  peat,  disregarding  sulphur,  which  is  seldom 
present,  at  least  in  quantity  sufficient  to  have  any  appreciable  influence,  may  be 
taken  to  be  as  given  in  Table  No.  3 1 . 

Table  No.  31. — Average  Composition  of  Irish  Peat. 


CONSTITUENTS. 

Perfectly  Dry. 

Including  25  per  cent 
of  Moisture. 

Including  30  per  cent 
of  Moisture. 

Carbon, 

59-o 

44.0 

41.2 

Hydrogen, 

6.0 

4-5 

4-2 

Oxygen, 

30.0 

22.5 

21.0 

Nitrogen, 

1.25 

I.O 

0.8 

Ash, 

4.0 

3-° 

2.8 

Moisture, 



25.0 

30.0 

By  calculation  the  thermal  value  of  dry  Irish  peat  of  the  average  composition, 
shown  in  Table  No.  31,  is- — 

Carbon      -     .      .    .-        .       14,650  X  0.59  =8,643.5     B.  T.  U. 

Hydrogen      .          .          62,100  (.06 — ^)  =  1,397.25  B.  T.  U. 
Total  B.  T.  U 10,040.75 


44 


MECHANICAL    DRAFT. 


Coal.  —  The  extensive  distribution,  the  portable  character  and  the  heat  value 
of  coal  make  it  the  principal  fuel  of  all  civilized  nations.  Coal  is  in  effect  the 
reservoir  of  the  stored  energy  of  the  sun,  by  the  action  of  whose  heat  rays  it 
was  produced.  It  is  a  fossil  fuel  for  whose  existence  geology  thus  accounts : 
During  that  period  of  the  earth's  formation  known  as  the  carboniferous  age, 
vegetation  was  rank  in  the  extreme.  The  atmosphere  contained  an  amount  of 
carbonic  acid  far  in  excess  of  that  now  present.  The  presence  in  the  atmos- 
phere of  this  excess  of  carbon,  which  is  the  food  of  the  plants,  as  well  as  the 
temperature  and  the  climatic  conditions,  were  all  favorable  to  the  most  prolific 
development  of  plant  life.  Age  after  age  was  employed  by  this  vegetable 
growth  in  freeing  the  atmosphere  from  carbonic  acidf  and  in  storing  up  the 
potential  energy  of  the  sunlight  as  woody  fibre  in  the  form  of  carbon,  separated 
from  oxygen.  By  this  continuous  process  of  growth  and  death  of  vegetable 
matter  the  earth  became  strewed  with  the  remains,  which  were  gradually  com- 
pacted into  peat  beds  of  enormous  extent.  With  succeeding  climatic  and  geo- 
logical changes,  these  peat  beds,  one  after  another,  become  submerged  and 
overlaid  by  thousands  of  feet  of  sandstone,  limestone  and  slate.  Under  the 
tremendous  pressure  thus  exerted  the  peat  beds  were  compressed  and  converted 
by  successive  stages  into  lignite,  brown  coal,  gaseous  coal,  bituminous  coal  and 
semi-anthracites.  In  certain  localities,  through  some  igneous  agency,  the  coal 
beds  were  subjected  to  distillation,  the  hydro-carbons  in  the  forms  of  oil  or  gas 
being  thus  driven  away  to  be  stored  in  the  natural  reservoirs  of  the  earth,  while 
anthracite,  shiny  and  rock-like,  was  left  behind. 

It  must  be  obvious  that  sharp  lines  of  demarcation  between  the  various  kinds 
of  coal  cannot  exist',  and,  therefore,  that  they  can  only  be  approximately  classi- 
fied, for  one  form  merges  into  another.  A  fair  illustration  of  the  different  stages 
in  the  process  of  alteration  of  wood  fibre  into  anthracite  coal  is  presented  in 
Table  No.  32. 

Table  No.  32. — Conversion  of  Wood  Fibre  into  Anthracite. 


DESCRIPTION. 

Carbon. 

Hydrogen. 

Oxygen. 

Wood  fibre  (cellulose), 

52-65 

5-25 

42.10 

Peat, 

60.44 

5-96 

33-60 

Lignite, 

66.96 

5-27 

27.76 

Lignite  (brown  coal), 

74.20                     '        5.89                            19.90 

Coal  (bituminous), 

76.18                              5.64                            18.07 

Coal  (semi-anthracite), 

90.50                              5.05                              4.40 

Anthracite, 

93.85                              3-96 

3-'9 

MECHANICAL    DRAFT.  45 

Coals  are  usually  classified  according  to  the  amounts  of  carbon  and  volatile 
matter  which  are  present  in  their  composition,  although  different  methods  are 
adopted  by  different  authorities.  The  following  is  the  classification  generally 
adopted,  beginning  with  those  containing  the  greatest  proportion  of  carbon:  — 

Anthracites   .  \  Hard  anthracites. 

(  Semi  or  gaseous  anthracites. 

{Semi-bituminous          j  Semi-bituminous  cherry  coal, 
coals          .         .  t  Semi-bituminous  splint  coal. 

<  Caking  coal. 
Bituminous  J  Cherry  coal. 

coal  .         .  (  splint  coal. 

(  Cannel  coal. 

Hydrogenous  or  gas  coal       .          .  -<  Hydrogenous  shaly  coal. 

(  Asphaltic  coal. 

Lignite. 

The  general  composition  of  these  coals  has  already  been  given  in  Table  No. 
13.  In  the  consideration  of  their  characteristics  they  will  be  taken  up  in  the 
order  of  their  geological  formation.  Their  progressive  alteration  from  wood  to 
coal  is  thus  clearly  indicated. 

Lignite.  —  Although  classed  among  mineral  coals,  from  a  geological  stand- 
point, lignite  properly  occupies  a  position  between  peat  and  bituminous  coal. 
It  is  believed  to  be  of  later  origin  than  bituminous  coal,  and  is  in  a  less 
advanced  stage  of  decomposition.  The  woody  fibre  and  vegetable  texture  of 
lignite  are  almost  entirely  wanting  in  coal,  although  there  is  little  question  as 
to  their  common  origin.  Although  much  like  brown  coal  in  general  appearance, 
lignite  differs  from  it  in  the  fact  that  upon  distillation  it  yields  acetic  acid,  while 
brown  coal  produces  only  ammoniacal  liquor.  Like  peat,  lignite  presents  much 
variety  in  appearance,  some  specimens  being  almost  as  hard  as  true  coal,  while 
others  possess  a  distinctly  woody  structure  and  are  of  a  light-brown  color.  It 
has  an  uneven  fracture  and  a  dull  and  somewhat  fatty  lustre.  Being  easily 
broken,  it  will  not  readily  bear  transportation,  while  exposure  to  the  weather 
causes  it  to  rapidly  absorb  moisture  and  to  crumble  easily.  Its  value  as  a  fuel 
is,  therefore,  limited,  for  it  must  be  used  near  its  place  of  occurrence,  and  very 
soon  after  it  is  mined.  It  is  non-caking  and  yields  but  moderate  heat,  being 
inferior  to  even  the  poorer  varieties  of  bituminous  coal.  In  this  country  its  use 
is  decidedly  limited,  being  restricted  to  the  locality  of  the  mines  which  produce 
it.  It  is  plenteous,  however,  west  of  the  Mississippi,  in  which  territory  it  is  used 


MECHANICAL    DRAFT. 


to  a  considerable  extent.  The  three  analyses  which  are  presented  in  Table  No. 
33  giye  the  average  composition  of  samples  from  the  widely  separated  states  of 
Kentucky,  Washington  and  Colorado. 

Table  No.  33.  —  Composition  of  Lignite. 


LOCALITY. 

Specific 
Gravity. 

Fixed 
Carbon. 

Volatile 

Combusti- 
ble Matter 

Water. 

Ash. 

Total 
Volatile 
Matter. 

Coke. 

Kentucky, 

1.  201 

40.0 

23.0 

30.0 

7-0 

53-o 

47-0 

Washington, 

52.85 

31-75 

7.00 

3.00 

61.25 

38-75 

Colorado, 

I.27I 

4I-25 

46.00 

3-5° 

9-25 

50.50 

49-5° 

Bituminous  Coal.  —  The  classification  of  bituminous  coal  is  rendered  difficult 
because  of  the  lack  of  definite  lines  of  demarcation  between  the  varieties.  As 
a  rule,  however,  coal  containing  as  much  as  18  to  20  per  cent  of  volatile  com- 
bustible is  called  bituminous.  Some  bituminous  coal  yields,  upon  analysis,  as 
much  as  50  per  cent  of  volatile  matter  and  sometimes  more.  In  proximate 
composition, —  namely,  in  fixed  carbon,  volatile  matter  and  earthy  matter, —  the 
bituminous  coals  maybe  regarded  as  ranging  between  the  following  general 
limits  :  — 

Fixed  carbon 52  to  84  per  cent. 

Volatile  matter    .          .         .         .         .          12  to  48       " 
Earthy  matter       .          .          .          .          .  2  to  20        " 

Sulphur       .         .         .         .          .          .  i  to    3        " 

The  amount  of  water  expelled  by  heating  to  212°  is  from  i  to  4  per  cent. 
In  ultimate  composition,  as  shown  by  refined  analysis,  the  approximate  range 
of  composition  is  as  follows  :  — 

Carbon  .          .         .          .         .          .  75  to  80  per  cent. 

Hydrogen  .          .          .         .         .          .  5  to    6        " 

Nitrogen  .          .  .         .         .  i  to    2        " 

Oxygen 4  to  10        " 

Sulphur 0.4  to    3        " 

Ash     .  .         .         .         ,  .  3  to  10       " 

In  its  external  properties,  ordinary  bituminous  coal  varies  in  color  from  a 
pitch  black  to  a  dark  brown,  with  a  lustre  that  is  vitreous  or  resinous  in  the 
more  compact  specimens,  and  silky  in  those  showing  traces  of  vegetable  fibres. 
Irrespective  of  natural  joints,  the  fracture  of  bituminous  coal  is  generally  con- 
choidal.  The  distinctive  characteristic  of  this  fuel  is  the  emission  of  yellow 
flame  and  smoke  when  burning. 


MECHANICAL    DRAFT. 


47 


All  bituminous  coals  may  be  classified  on  broad  lines  as  either  caking  or  non- 
caking. 

Caking  Coal  is  the  name  given  to  any  coal  which,  when  heated,  seems  to  fuse 
together  and  swell  in  size,  becomes  pasty  in  appearance  and  emits  a  sticky  sub- 
stance over  the  surface,  while  liberating  small  streams  of  gas  which  burn  with 
a  bright  yellow  or  reddish  flame  terminating  in  smoke.  It  is  characteristic  of 
caking  coal  that  the  pasty  lumps  will  cohere  in  the  fire  and  form  spongy-looking 
masses,  not  infrequently  covering  the  entire  surface  of  the  grate.  Such  coals, 
unusually  rich  in  volatile  hydro-carbons,  are  considered  most  valuable  for  gas 
manufacture. 

Non-Caking  Coal  has  the  property  of  burning  freely  in  the  fire ;  hence  the 
common  appellation,  "  free-burning  coal."  The  heat  does  not  cause  the  lumps 
to  fuse  or  run  together.  The  block  coal  of  the  Western  States  is  a  representative 
non-caking  coal.  It  consists  of  successive  layers  which  are  easily  separated 
into  thin  slices.  The  surfaces  which  are  thus  displayed  are  generally  covered 
with  a  layer  of  very  finely  divided  fibrous  carbon  and  are  dull  and  lustreless. 
When  coal  of  this  character  is  broken  at  right  angles  to  this  lamination  the  sur- 
face is  bright  and  glistening. 

The  ultimate  composition  of  various  bituminous  coals  is  given  in  Table  No. 
13,  which  has  already  been  presented;  while  among  the  coals  listed  in  Table 
No.  36,  which  follows  on  succeeding  pages,  are  also  many  that  would  be  classed 
as  bituminous. 

Cannel  Coal  is  a  variety  of  bituminous  coal  very  rich  in  carbon.  It  kindles 
readily,  burns  without  melting  and  emits  a  bright  flame  like  that  of  a  candle. 
It  differs  greatly  in  appearance  from  all  other  bituminous  coals,  being  very 
homogeneous,  having  a  dull,  resinous  lustre,  and  breaking  without  following 
any  distinct  line  of  fracture.  It  is  exceedingly  valuable  as  a  gas  coal  because 
of  its  richness  in  hydro-carbons,  but  is  little  used  in  this  country  as  a  steam  or 
boiler  coal.  The  proximate  analysis  of  a  few  typical  American  specimens  is 
presented  in  Table  No.  34. 

Table  No.  34.  —  Composition  of  Cannel  Coal. 


LOCALITY. 

Specific  Gravity. 

Fixed  Carbon. 

Volatile  Matter. 

Earthy  Matter. 

Franklin,  Pa., 

40.13 

44.85 

15.02 

Dorton's  Branch,  Ky., 
Breckenridge,  Ky., 
Davis  County,  Ind., 

1.25 
1.23 

55-1 
32.0 
42.0 

42.9 

55-7 
52.0 

2.0 
12.3 
6.0 

MECHANICAL    DRAFT, 


Semi-Bituminous  Coal  is  softer  and  contains  more  volatile  matter  than  true 
anthracite  coal,  but  in  its  general  characteristics  closely  approaches  that  fuel. 
It  resembles  in  appearance  the  anthracites  more  closely  than  it  does  the  bitu- 
minous coals,  but  its  fracture  is  less  conchoidal  than  that  of  the  former ;  it  is 
lighter  and  both  kindles  and  burns  more  rapidly.  Because  of  this  latter  feature 
it  is  extremely  valuable  as  a  fuel,  for  when  burned  it  readily  gives  off  a  great 
quantity  of  heat  and  can  always  be  relied  upon  to  keep  up  an  intense  and  free- 
burning  fire  requiring  comparatively  little  attention,  readily  cleaned  and  kept  in 
good  condition.  It  is,  when  pure,  almost  entirely  free  from  smoke  and  soot. 

The  proximate  analysis  of  semi-bituminous  coal  from  Cumberland,  Md., 
and  Blossburg,  Pa.,  is  given  in  Table  No.  35. 

Table  No.  35.  —  Composition  of  Semi-Bituminous  Coal. 


LOCALITY. 

Specific  Gravity. 

Fixed  Carbon. 

Volatile  Matter. 

Sulphur. 

Earthy  Matter. 

Cumberland,  Md., 
Blossburg,  Pa., 

1.41 

I.32 

68.44 

73-" 

17.28 
15.27 

0.71 
0.85 

13.98 
10.77 

Semi- Anthracite  CoaL —  Among  the  semi-anthracite  coals  are  classed  those 
which  contain  from  7  to  8  per  cent  of  volatile  combustible  matter.  Because  of 
the  presence  of  this  ingredient,  which  apparently  exists  in  the  gaseous  state  in 
the  cells  or  cracks  of  the  coal,  this  variety  kindles  more  readily  and  burns  more 
rapidly  than  hard  anthracite.  Analysis  of  Wilkesbarre,  Pa.,  semi-anthracite, 
which  is  compact,  conchoidal,  iron  black  and  shiny,  shows  the  following  to  be 
its  composition  :  — 


Fixed  carbon 
Volatile  matter 
Earthy  matter 


88.90  per  cent. 
7.68       « 
3-49      '" 


100.07 

Its  specific  gravity  is  1.4. 

Anthracite  Goal.  —  Pure  anthracite,  sometimes  called  blind  coal,  ignites 
slowly,  is  a  poor  conductor  of  heat,  and  burns  at  a  very  high  temperature. 
When  pure,  it  consists  of — 


Carbon 

Hydrogen 

Oxygen  and  nitrogen 

Water 

Ash 


90  to  94  per  cent, 
i  to     3        " 
i  to    3        " 
i  to     2        " 
3  to     4        " 


MECHANICAL   DRAFT.  49 

It  is  thus  evident  that  it  is  composed  almost  entirely  of  carbon :  in  fact,  this 
is  its  distinguishing  characteristic.  The  hydro-carbons,  as  evidenced  in  the 
volatile  constituents,  are  present  in  very  small  proportion.  As  a  consequence, 
it  is  not  a  long-flaming  coal,  but  when  in  a  state  of  incandescence  its  radiant 
power  is  great,  owing  to  the  intensity  of  combustion  of  the  practically  pure  car- 
bon of  which  it  consists. 

In  the  process  of  burning  it  neither  swells,  softens  nor  gives  off  smoke.  The 
flame  is  quite  short,  of  a  yellowish  tinge,  changing  to  a  faint  blue,  and  largely 
due  to  the  presence  of  water  which  is  decomposed  by  the  heat.  This  flame  is 
free  from  particles  of  solid  carbon,  and  has  the  appearance  of  being  transparent. 
Anthracite  coal  is  homogeneous  in  structure ;  its  fracture  is  decidedly  con- 
choidal,  and  it  is  but  slightly  affected  by  exposure  to  the  weather. 

Analysis  of  anthracite  coal  from  Tamaqua,  Pa.,  shows  it  to  consist  of  — 

Carbon 92.07  per  cent. 

Volatile  matter            .....         5-°3       " 
Earthy  matter 2.90       " 

100.00 
and  to  have  a  specific  gravity  of  1.57. 

Geographical  Classification.  —  Although  widely  distributed  throughout  the 
United  States,  the  various  kinds  of  coal  may  be  geographically  classified  in  a 
general  manner,  as  follows  :  — 

(  Eastern  portion  of  Allegheny  Mountains 
Anthracite  \ 

(       and  Rocky  Mountains  of  Colorado. 

f  Caking,  Mississippi  Valley. 

Bituminous     J  Non-caking,       Maryland  and  Virginia. 

coals,  ^  Cannel,  Pennsylvania,  Indiana  and  Missouri. 

Lignites  ....  Colorado,  Kentucky  and  Washington. 
A  carefully  selected  list  of  analyses  of  representative  American  coals,  geo- 
graphically arranged,  is  presented  in  Table  No.  36',  which  makes  clear  the 
differences  which  exist  even  in  coals  from  the  same  locality.  The  ultimate 
value  of  any  coal  as  a  steam  producer  must  be  measured  by  the  amount  of  water 
it  can  evaporate  when  properly  burned  in  the  furnace  of  a  steam  boiler.  But 
this  value  may  be  modified  by  certain  characteristics  of  the  coal ;  and,  as  will 
be  pointed  out  later,  the  efficiency  of  a  fuel  is  to  a  considerable  extent  depend- 
ent upon  the  character  of  the  boiler  and  furnace  in  connection  with  which  it  is 
consumed. 


'Helios.     E.  D.  Meier.     St.  Louis,  1895. 


5o  MECHANICAL    DRAFT. 

Table  No.  36. — Composition  and  Fuel  Value  of  American  Coals. 


COAL, 

NAME   OR    LOCALITY. 

I 

Constituents  in  per  cent  of  Total  Weight. 

Fuel  Value  per  Pound 
of  Coal. 

t 

.2 

1 

2 

2  i; 
11 

£S 

3 

il 

EJ 

4 

1 

5 

I 
1 

6 

B.  T.  U. 

Calculated. 

£s 
z>  e 

•'Z 

?*£ 

«c3 

8 

is]* 

HIS! 
9 

ARKANSAS. 

. 

Coal  Hill,  Johnson  Co., 

i-35 

'4-93 

74.06 

9.66 

3-04 

!3'7I3 

I4.I 

Coal  Hill,  Johnson  Co., 

1.70 

14.60 

74.91 

8.79 

3-04 

11,812 

12.22 

Huntington  Co., 

1.30 

18.95 

7I-51 

8.24 

0.78 

11,756 

12.17 

Huntington  Co., 

1.30 

18.90 

73-15 

6.65 

o-75 

11,907 

12.32 

Huntington  Co., 

1.27 

18.89 

71-74 

8.10 

0.65 

12,537 

12-97 

Lignite, 

9.215 

9-54 

Jenny  Lind,  Sebastian  Co., 

1.26 

17.64 

72-48 

8.62 

2.  II 

13.964 

14.4 

Spadra,  Johnson  Co., 

1.47 

13-27 

78.63 

6.63 

1.  60 

14,420 

14.9 

COLORADO. 

Lignite, 

13.560 

14.04 

Lignite, 

13.865 

M-35 

Lignite,  slack, 

14.80 

32.00 

42.86 

10.34 

0.76 

8,500 

8.80 

Lignite,  slack,  North  Colorado, 

1  8.88 

3r-74 

40.08 

9-30 

c.6i 

Rouse  Mine, 

3-!3 

37-32 

30.00 

8.25 

ILLINOIS. 

Big  Muddy,  Jackson  Co., 

7-39 

28.28 

53-87 

10.46 

0.98 

11,466 

11.87 

Big  Muddy,  Jackson  Co., 

6.12 

30-95 

53-74 

9.19 

1.22 

11,529 

"•93 

Big  Muddy,  Jackson  Co., 

5-85 

31.84 

55-72 

6-59 

2.92 

11,781 

12.19 

Big  Muddy,  Jackson  Co., 

6-35 

31-5° 

55-25 

6.90 

2.02 

12,567 

13.0 

Bureau  Co., 

13.025 

13.48 

Colchester, 

1  1.  60 

25.02 

44.76 

18.62 

9,848 

10.19 

Colchester,  slack, 

5-3° 

25-45 

38-15 

31.10 

1.20 

9.035 

9-35 

Collinsville,  Madison  Co., 

9.20 

45-89 

3'-57 

13-34 

5-34 

10,143 

10.50 

Dumferline,  slack, 

9.64 

28.86 

39-48 

22.02 

9,401 

9-73 

Duquoin  Jupiter,  Perry  Co., 

11.30 

30-31 

49.91 

8.48 

0.91 

10,710 

1  1.  08 

Ellsworth,  Macoupin  Co., 

9.26 

42.22 

42.17 

6-35 

2.62 

12,175 

12.60 

Gillespie,  Macoupin  Co., 

12.61 

30.58 

45-27 

"•54 

i-45 

9.739 

10.09 

Girard,  Macoupin  Co., 

9.70 

34-39 

45-76 

10.15 

3-49 

9.954 

10.30 

Girard,  Macoupin  Co., 

8.90 

32.25 

42.89 

15.96 

8.10 

10,269 

10.63 

Heitz  Bluff,  St.  Clair  Co., 

8-95 

37-8i 

48.24 

5.00 

3-27 

10,332 

10.69 

Johnson's,  St.  Clair  Co., 

5-5o 

40.14 

40-53 

13-83 

4.80 

".723 

12.10 

Loose's,  Sangamon  Co., 

10.71 

37.62 

45-°7 

6.60 

2-39 

".479 

II.9 

Mercer  Co., 

i3.I23 

I3-58 

MECHANICAL   DRAFT. 


COAL. 

NAME   OR    LOCALITY. 

Constituents  in  per  cent  of  Total  Weight. 

Fuel  Value  per  Pound 
of  Coal. 

"  Moisture. 

If 

3 

4 

1 

5 

o  Sulphur. 

W(j 
7 

B.  T.  U.  by 
Calorimeter. 

8.1  1* 

in 

HH.=  § 

9 

ILLINOIS.  —  Concluded. 

Montauk  Co., 

12,659 

13.10 

Mt.  Olive,  Macoupin  Co., 

10.38 

36.68 

46.10 

6.84 

3-53 

11,763 

Oakland,  St.  Clair  Co., 

8-30 

34-40 

43.12 

14.18 

4.42 

!0,395 

10.76 

Reinecke,  St.  Clair  Co., 

7.56 

39.81 

42.49 

10.14 

4.02 

II,72O 

12.  1 

Riverton,  Sangamon  Co., 

1  1.  06 

37-94 

42.98 

8.02 

3-27 

11,406 

1  1.8 

St.  Clair, 

7.80 

30-69 

39-68 

21.83 

9.62 

9,261 

9.58 

St.  Clair, 

10.25 

33.10 

41.79 

14.86 

6.92 

10,294 

10.65 

St.  Clair, 

11.15 

34-19 

44-94 

9.72 

4.27 

10,647 

11.02 

St.  Bernard, 

14.36 

30.86 

48.39 

6-39 

1.38 

I0,o8o 

10.44 

St.  John,  Perry  Co., 

9.82 

28-35 

45-77 

1  6.08 

2.06 

9,765 

10.10 

St.  John,  Perry  Co., 

13.60 

24.46 

43-54 

15.40 

1.83 

9,828 

10.18 

Streator,  La  Salle  Co., 

12.01 

35-32 

48.78 

3-89 

2.38 

",403 

1  1.  80 

Trenton,  Clinton, 

r3-34 

30-39 

51.96 

4-31 

0.92 

10,584 

10.96 

Trenton,  Clinton, 

9-95 

31.04 

52-03 

6.98 

1.04 

11,245 

11.63 

Vulcan  Nut,  St.  Clair  Co., 

7-44 

30.86 

45-09 

1  6.6  1 

1.32 

9,45° 

9.78 

Vulcan  Nut,  St.  Clair  Co., 

10.30 

27.91 

48.99 

12.80 

0.71 

10,626 

11.00 

INDIANA. 

Block, 

3-5° 

32-5° 

63.00 

1.  00 

0.98 

14,020 

14.5 

Block, 

13,588 

14.38 

Caking, 

14,146 

14.64 

Cannel, 

INDIAN  TERRITORY. 

Atoka, 

6.66 

35-42 

5I-32 

6.60 

3-73 

11,088 

11.47 

Choctaw  Nation, 

1.59 

23-31 

66.85 

8.25 

1.18 

12,789 

13-23 

IOWA. 

Good  Cheer, 

10.85 

30-32 

3I-38 

27-45 

7-32 

8,702 

9.01 

KENTUCKY. 

Caking, 

14,391 

14.89 

Cannel, 

15,198 

16.76 

Cannel, 

1  3>36o 

13.84 

Lignite, 

9,326 

9.65 

MISSOURI. 

Bevier  Mines, 

9,890 

10.24 

MECHANICAL    DRAFT. 


COAL, 

Constituents  in  per  cent  of  Total  Weight. 

Fuel  Value  per  Pound 
of  Coal. 

5 
1 

Is 

•i! 

3 

jj 

£« 

4 

5 

1' 
3 

6 

7 

B.  T.  U.  by 
Calorimeter. 

9 

MARYLAND. 

Cumberland, 

12,226 

12.65 

George's  Creek, 

13,500 

13.98 

NEW  MEXICO. 

Coal, 

2-35 

35-53 

50.24 

11.88 

0.61 

11,756 

12.17 

OHIO. 

Briar  Hill,  Mahoning  Co., 

2.47 

31-83 

64.25 

1-45 

0.56 

I3.7M 

I4.2 

Hocking  Valley, 

8.25 

35-88 

53-J5 

2.72 

0.43 

*3»4M 

13-9 

PENNSYLVANIA. 

Anthracite, 

14,199 

14.70 

Anthracite, 

J3.535 

14.01 

Anthracite, 

14,221 

14.72 

Anthracite,  pea, 

2.04 

6.36 

78.41 

13.19 

12,300 

12-73 

Anthracite,  buckwheat, 

3.88 

3-84 

81.32 

10.96 

0.67 

12,200 

12.63 

Cannel, 

I3.M3 

13.60 

Connellsville, 

13,368 

13.84 

Pittsburgh  (average), 

i.  80 

35-34 

54-94 

7.92 

1.97 

I3.I04 

13.46 

Pittsburgh,  coking, 

i-43 

30.22 

61.87 

6.48 

J-35 

14,415 

14.9 

Youghiogheney, 

1.96 

34.06 

58-98 

5.00 

12,936 

'3-39 

Youghiogheney, 

2.02 

32.14 

58-96 

6.88 

0.88 

I2,6OO 

13.03 

Reynoldsville, 

1.20 

27.12 

65.88 

5-8o 

12,981 

13-44 

TENNESSEE. 

Glen  Mary,  Scott  Co., 

2.15 

3»-47 

61.63 

4-75 

0-94 

I3»l67 

13-63 

TEXAS. 

Fort  Worth, 

14.42 

30-03 

42-53 

13.02 

1.47 

9-45° 

9.78 

Fort  Worth, 

4.60 

347? 

49.27 

11.41 

1.56 

",403 

1  1.  80 

Lignite, 

12,962 

i3-4i 

WEST  VIRGINIA. 

New  River, 

14,200 

14.70 

New  River, 

13,400 

13-87 

New  River, 

o-94 

18.19 

75-89 

4.68 

0.30 

New  River, 

0.76 

18.65 

79.26 

i.  ii 

0.23 

MECHANICAL    DRAFT.  53 

Petroleum.  —  The  only  natural  liquid  fuel  is  crude  petroleum  oil.     This  is 
distinctly  a  hydro-carbon  liquid,  and  is  found  in  abundance  in  certain  localities 
in  America  and  Europe.     The  principal  sources  of  supply  are,  however,  in  the 
Ohio  Valley  of  the  United  States,  and  on  the  borders  of  the  Caspian  Sea  in 
Eastern  Europe.     As  already  explained,  it  appears  to  be  the  result  of  distilla- 
tion of  coal  under  great  pressure.     It  is  found  in  natural  cavities  beneath  the 
earth's  surface,  whence  it  is  either  pumped,  or  flows  to  the  surface  after  the 
manner  of  operation  of  an  artesian  well. 

Crude  petroleum  is  dark  brown  in  color,  with  a  perceptible  greenish  tinge, 
and  has  a  specific  gravity  which  averages  about  0.8.     It  is  composed  of  a  great 
number  of  liquid  hydro-carbons  varying  widely  in  specific  gravity  and  chemical 
composition,  and  each  separable  from  the  others  by  fractional  distillation.     The 
ultimate    analysis    of   an    average    sample  indicates    about    the  following  com- 
position :  — 

Carbon          .......          84  per  cent. 

Hydrogen     .          .          .          .          .          .          .          14        " 

Oxygen         .  2        " 

100  per  cent. 

Allowing  for  the  combination  of  the  inherent  oxygen  with  its  equivalent  of 
hydrogen  to  form  water,  the  practical  composition  becomes  — 

Carbon          .......     84      per  cent. 

Hydrogen     . 13.75        " 

Water 2.25        " 

100      per  cent. 

The  heat  value  of  a  pound  of  petroleum  of  the  above  composition  is,  there- 
fore, — 

Carbon      .         .         .         0.84    x    14,650  =    12,306  B.  T.  U. 
Hydrogen  .          .     0.1375    X    62,100  =     8,539        " 

20,845  B.  T.  U. 

Natural  Gas.  —  Although  springs  of  natural  gas  exist  in  nearly  every  State  of 
the  Union,  the  commercial  use  of  this  fuel  is  practically  limited  to  the  States 
of  Indiana,  Ne.w  York,  Pennsylvania  and  Ohio,  in  which  the  largest  supply  is 
to  be  found.  This  gas  is  an  inseparable  companion  of  natural  oil  or  petroleum, 
as  would  be  the  natural  consequence  of  their  both  being  the  products  of  dis- 
tillation of  coal.  In  its  composition  natural  gas  varies  greatly.  Not  only  is 
there  a  marked  difference  in  composition  between  the  gas  from  different  wells, 
but  also  between  samples  which  are  taken  at  different  times  from  the  same  well. 


54 


MECHANICAL    DRAFT. 


The  variation  in  the  composition  of  natural  gas  is  illustrated  by  the  results  of 
six  analyses,  made  within  a  period  of  three  months,  of  different  samples  from 
a  well  near  Pittsburg,  Pa.,  as  presented  in  Table  No.  37. 

Table  No.  37.  —  Variation  in  Composition  of  Natural  Gas. 


CONSTITUENTS. 

i 

2 

4 

5 

.     6 

Marsh  gas, 

57.85 

75-16 

72.l8 

65-25 

60.70 

49-5S 

Hydrogen, 

9.64 

14.45 

20.02 

26.16 

29.03 

35-92 

Ethylic  hydride, 

5.20 

4.80 

3-60 

5-5° 

7.92 

12.30. 

Olefiant  gas, 

0.80 

0.60 

O.JO 

0.80 

0.98 

0.60 

Oxygen, 

2.10 

1.20 

1.  10 

0.80 

0.78 

0.80 

Carbonic  oxide, 

1.  00 

0.30 

1.  00 

0.80 

0.58 

0.40 

Carbonic  acid, 

0.00 

0.30 

0.80 

0.60 

0.00 

0.40 

Nitrogen, 

23.41 

2.89 

o.oo 

o.oo 

0.00 

0.00 

Analyses  from  various  wells  in  Indiana  and  Ohio  indicate   the  composition 
to  be  as  given  in  Table  No.  38. 

Table  No.  38.  —  Composition  of  Natural  Gas  from  Ohio  and  Indiana. 
OHIO. 


CONSTITUENTS. 

Fostoria. 

Findlay. 

St.  Mary's 

Muncie. 

Anderson 

Kokomo. 

Marion. 

Hydrogen, 

1.89 

1.64 

1.94 

2-35 

1.86 

1.42 

1.20 

Marsh  gas, 

92.84 

93-35 

93-85 

92.67 

93-07 

94.16 

93-57 

Olefiant  gas, 

0.2O 

o-35 

O.2O 

0.25 

0.47 

0.30 

0.15 

Carbonic  oxide, 

0-55 

0.41 

0.44 

0-45 

0-73 

0.55 

0.60 

Carbonic  acid, 

0.20 

0.25 

0.23 

0.25 

0.26 

0.29 

0.30 

Oxygen, 

o-35 

o-39 

°-35 

o-35 

0.42 

0.30 

o-55 

Nitrogen, 

3.82 

3-4i 

2.98 

3-53 

3.02 

2.80 

3-42 

Hydrogen  sulphide, 

0.15 

0.20 

0.21 

0.15 

.0.15 

0.18 

0.20 

Artificial  Fuels.  —  Although  artificial  fuels  serve  a  useful  purpose  in  steam- 
making,  their  use  is  by  no  means  as  extended  as  that  of  the  natural  fuels.  The 
desirability  of  employing  one  in  preference  to  the  other  is  dependent  largely 
upon  financial  considerations  ;  and  that  fuel  is  to  be  chosen  which,  other  things 
equal,  will  evaporate  the  most  water  for  a  given  total  expenditure.  Artificial 
fuels  may  be  broadly  classified  under  the  headings  charcoal,  coke,  fuel  gas  and 
patent  fuels. 


MECHANICAL   DRAFT. 


55 


Charcoal.  —  Wood,  protected  from  the  atmosphere  and  heated  at  about  600°, 
gives  up  its  gaseous  or  volatile  elements,  and  is  converted  into  charcoal.  The 
best  charcoal  consists  almost  entirely  of  pure  carbon ;  but  in  so  far  as  the  proc- 
ess of  manufacture  falls  short  of  perfection,  so  the  proportion  of  carbon  is 
reduced.  Charcoal  is  distinctly  the  result  of  a  process  of  carbonization ;  and 
under  the  condition  of  distillation  in  vessels  externally  heated  to  various  tem- 
peratures, its  quality  is  improved  as  its  temperature  is  increased.  The  results 
of  this  process  when  applied  to  black  alder,  previously  dried  at  about  300°,  are 
presented  in  Table  No.  39. 

Table  No.  39.  —  Composition  of  Carbon  Produced  at  Various  Temperatures. 


CONSTITUENTS  OF  THE  SOLID  PRODUCT. 

Temperature  of 

Carbonization. 

Carbon.                    Hydrogen. 

Nitrogen 
and  Loss. 

Ash. 

302°  F.                   47.51 

6.12 

46.29 

0.08 

47-51 

392                                 51.82 

3-99 

43-98 

0.23 

39-88 

482                                 65.59 

4.8l 

28.97 

0.63 

32.98 

592                                 73-24 

4-25 

21.96 

o-57 

24.61 

662                                 76.64 

4.14 

18.44 

0.6  1 

22.42 

Sio                        81.64 

4.96 

15.24 

1.61 

15.40 

1.^73 

81.97 

2.30 

I4-I5 

i.  60                     15.30 

Peat  charcoal,  produced  by  the  carbonization  of  ordinary  air-dried  peat,  is 
very  friable  and  porous,  and  extremely  difficult  to  handle  without  reducing  it  to 
very  small  particles  almost  powdery  in  their  character.  Although  it  is  easily 
ignited  and  burns  readily,  its  physical  characteristics  are  such  as  to  prevent 
its  general  use. 

Coke.  —  The  residual  product  of  the  carbonization  of  bituminous  coal  is 
known  as  coke.  By  this  process  the  hydro-carbon  gases  are  expelled,  and  the 
coal  is  reduced  to  a  substance  somewhat  porous  in  its  character  and  consisting 
almost  entirely  of  carbon.  The  coke  produced  by  the  partial  combustion  of 
coal  in  coke  ovens  is  dark  gray  in  color,  hard,  porous  and  brittle,  with  a  slightly 
metallic  lustre.  That  resulting  as  a  by-product  from  the  distillation  of  gas  in 
the  retorts  of  gas  works  is  not  so  hard,  ignites  more  readily  and  burns  with  a 
draft  less  intense  than  that  required  for  the  combustion  of  coke  which  has  been 
formed  by  the  first  method.  It  is,  therefore,  better  adapted  as  a  fuel  for  steam- 
boiler  furnaces. 

The  quality  of  such  coke  is  affected  by  the  temperature  and  by  the  pressure 
under  which  distillation  takes  place.  From  experiments  by  Mr.  A.  L.  Steavenson, 


5 6  MECHANICAL   DRAFT. 

reported  to  the  Iron  and  Steel  Institute  at  Newcastle,  Eng.,  it   appeared  that 
with  a  furnace  of  special  construction  and  coal  of  the  following  composition,  — 

Oxygen -        .  *  '  6.7  percent. 

Carbon 84.9        " 

Hydrogen     .          .         .         .         .         .         .         4.5         " 

Nitrogen       .         .         .         .         .         .         .          i.o         " 

Sulphur         . 0.6         " 

Ash 2.3 

the  yield  was  about  60  per  cent  of  coke  of  the  following  composition :  — 

Carbon 96.2  per  cent. 

Ash .         .         3.8 

Hence  the  composition  and  relative  weight  of  the  materials  lost  in  coking 
were  — 

Carbon  .         .  .         .         .         .       68.1  per  cent. 

Hydrogen  .          .         .         .         .          .  11.2        " 

Nitrogen  .         .          .         .         .         .         .         2.5        " 

Sulphur  .          .         .          .         .         .          ,i.6'" 

Oxygen  .......       16.6        " 

Fuel  Gas.  —  Although  carbonic  oxide  and  hydrogen  are  combustibles,  the 
production  of  which  is  incident  to  the  combustion  of  all  fuels,  they  are  never 
independently  manufactured  for  use  as  fuels.  But  hydrogen  and  carbon,  asso- 
ciated in  the  form  of  volatile  hydro-carbons,  serve  most  excellently  the  purposes 
of  fuels,  although  their  cost  must  determine  their  efficiency.  Notwithstanding 
the  fact  that  illuminating  gas  made  from  bituminous  coal  by  distillation  in 
retorts  has  been  in  common  use  for  nearly  a  century,  the  idea  of  directly  convert- 
ing a  solid  fuel  into  one  of  gaseous  form  for  its  readier  utilization  for  producing 
heat  has  only  been  carried  into  practice  during  the  past  twenty-five  or  thirty 
years.  Gas  as  a  fuel  first  appeared  in  the  form  of  "  producer  "  gas,  and  was 
primarily  introduced  for  metallurgical  purposes.  In  its  manufacture,  air,  mixed 
with  water  vapor,  was  passed,  under  powerful  pressure,  through  a  thick  bed  of 
burning  coal.  As  a  result  the  coal  was  only  burned  to  carbonic  oxide,  while 
the  watery  vapor  was  decomposed  so  that  the  resulting  gas  from  the  producer 
was  a  mixture  of  about  one-half  nitrogen  and  one-fourth  carbonic  oxide,  with 
varying  proportions  of  hydrogen  and  hydro-carbons.  Its  composition  depends 
upon  the  proportions  of  the  elements  in  the  original  fuel.  This  process,  how- 
ever, inherently  consumes  about  one-third  of  the  total  calorific  value  of  the  fuel, 
thereby  reducing  by  that  amount  the  resultant  heating  power. 


MECHANICAL    DRAFT. 


57 


Water  gas  is  the  result  of  a  somewhat  similar  process,  which  differs  princi- 
pally from  that  employed  in  the  manufacture  of  producer  gas  in  that  it  is  inter- 
mittent, first  air  and  then  steam  being  forced  through  a  bed  of  incandescent 
fuel.  For  illuminating  purposes  this  gas  is  carburetted,  so  that  it  actually 
exceeds  by  volume  the  value  of  coal  gas. 

Including  natural  gas  the  relative  volumes  and  weights  of  gaseous  fuels  are  :  — 

By  Weight.  By  Volume. 

Natural  gas,  1,000  1,000 

Coal  gas,  949  666 

Water  gas,  292  292 

Producer  gas,  76.5  130 

By  weight  and  volume  the  composition  of  these  gases  is  given  in  Table  No.  40. 
Table  No.  40. — Composition  of  Fuel  Gases. 


CONSTITUENTS. 

BY  VOLUME. 

BY  WEIGHT. 

Natural 
Gas. 

Coal  Gas. 

Water 
Gas. 

Producer 
Gas. 

Natural 
Gas. 

Coal  Gas. 

Water       Producer 
Gas.            Gas. 

Hydrogen, 

2.18 

46.0 

45-o 

6.0 

0.268 

8.21 

5.431          0.458 

Marsh  gas,                       |  92.60 

40.0 

2.0           3.0 

90-383 

57.20 

1.931          1.831 

Carbonic  oxide, 

0.50 

6.0 

45-o         23-5 

0.857 

15.02 

76.041       25.095 

Olefiant  gas, 

0.3I 

4.0 

0.0                 0.0 

0-531 

I  O.O  I 

O.OOO          O.OOO 

Carbonic  acid, 

0.26 

0.5 

4.0        1.5 

0.700 

1.97 

10.622        2.517 

Nitrogen, 

3.61 

J-5 

2.0 

65.0 

6.178         3.75 

3.380    69.413 

Oxygen, 

0-34 

°-5 

0.5           o.o 

0.666        1.43         0.965        o.ooo 

Watery  vapor,                      o.oo 

'•5 

i-5 

1.0 

o.ooo 

2.41 

1.630       0.686 

Sulphydric  acid,                   0.20 





0.417 

.                        j 

In  this  table  the  natural  gas  was  from  Findlay,  Ohio,  the  coal  gas  was  prob- 
ably an  average  sample  purified  for  illuminating  purposes,  the  water  gas  was 
made  for  heating  and  consequently  unpurified,  and  the  producer  gas  was  made 
from  anthracite  at  the  Pennsylvania  Steel  Works. 

Patent  Fuels.  —  Under  this  title  may  be  classed  a  large  variety  of  prepared 
fuels,  consisting  in  the  main  of  the  particles  of  some  finely  divided  combusti- 
ble pressed  and  cemented  together  by  a  substance  possessing  the  necessary 
adhesive  and  inflammable  properties. 

In  the  process  of  coal  mining,  sorting  and  shipping,  a  considerable  amount 
is  broken  into  fragments,  too  small  for  ordinary  commercial  use.  This  refuse, 
commonly  denominated  "  culm,"  possesses  practically  the  same  calorific  value 
as  the  coal  of  which  it  originally  formed  a  part;  but  its  finely  divided  character 


58  MECHANICAL    DRAFT. 

is  not  conducive  to  its  successful  use  in  an  ordinary  boiler  furnace.  It  may, 
however,  by  special  machinery,  be  mixed  with  sufficient  pitch  or  coal  tar,  and 
moulded  into  lumps  of  desirable  size. 

In  this  country  the  relative  price  of  coal  is  so  low,  as  compared  with  the  cost 
of  manufacture  of  such  pressed  fuel,  that  the  financial  return  hardly  warrants 
the  attempt  to  thus  utilize  the  culm.  In  France,  and  some  other  European 
countries,  fuel  of  this  character,  in  the  form  of  "briquettes,"  is  regularly  made 
of  coal  dust, —  bituminous  and  semi-anthracite, —  and  quite  extensively  used. 
To  some  extent  the  slow  progress  made  in  the  manufacture  of  briquettes  in  this 
country  is  doubtless  due  to  the  imperfect  systems  of  washing  and  jigging  which 
are  necessary  to  reduce  the  percentage  of  ash,  which  never  ought  to  exceed  10 
per  cent  in  such  fuel. 

The  attempt  has  also  been  made,  with  varying  success,  according  to  the  con- 
ditions, to  feed  the  coal  in  the  form  of  dust  directly  to  the  boilers,  by  forcing 
it  into  a  strong  air  current,  which  thus  spreads  it  throughout  the  furnace,  while 
at  the  same  time  furnishing  the  oxygen  necessary  for  combustion. 

By  means  of  glue,  tar,  pitch,  resin  and  the  like,  sawdust,  charcoal,  peat,  tan 
and  similar  refuse  have  been  cemented  together  for  use  as  a  fuel.  But,  except 
in  comparatively  few  instances,  the  cost  of  manufacture  of  prepared  fuels  has, 
in  this  country  at  least,  rendered  them  but  little,  if  any,  more  economical  than 
coal. 


CHAPTER   V. 
EFFICIENCY   OF  FUELS. 

Measure  of  Efficiency.  —  The  ultimate  efficiency  of  a  fuel  should  be  expressed 
in  the  total  amount  of  heat  it  is  capable  of  generating.  The  proportion  of 
that  heat  which  is  utilized  depends  upon  the  efficiency  of  the  boiler  or  other 
heat-abstracting  device.  Commercially,  however,  the  heat  value  of  fuels  is 
generally  measured  relatively  to  each  other,  and  expressed  in  the  number  of 
pounds  of  water  evaporated  per  pound  of  fuel.  In  practice,  the  physical  char- 
acter of  the  fuel,  the  form  and  construction  of  the  boiler  and  furnace,  the 
amount  of  air  supplied,  and  other  conditions,  have  an  important  influence  upon 
the  attainable  results.  In  fact,  the  effect  of  these  variables  is  such  as  to  render 
an  accurate  comparison  of  fuels  a  difficult  matter. 

In  their  ultimate  efficiency  they  may,  however,  be  considered  relatively  to 
each  other  or  to  an  established  standard.  As  carbon  is  the  most  important 
element  in  the  composition  of  all  fuels,  it  may  reasonably  be  selected  as  such 
a  standard.  For  the  purposes  of  comparison,  Table  No.  41  has  been  prepared 
to  show  the  efficiency  of  fuels  as  measured  in  thermal  units,  determined  by 
analysis  or  calorimetric  test  when  compared  with  pure  carbon  as  a  standard, 
having  a  thermal  value  of  14,650  B.  T.  U.  Measured  by  this  standard,  a  coal 
having  very  little  ash  and  a  large  amount  of  hydrogen  may,  because  of  its 
extremely  high  heating  power,  actually  show  an  efficiency  above  100  per  cent 
when  compared  with  carbon.  As  the  theoretical  efficiency  of  a  fuel  can  never 
be  realized  in  practice,  there  have  been  incorporated  in  this  table  the  fuel  effi- 
ciency and  the  number  of  pounds  of  water  evaporated  from  and  at  212°  per 
pound  of  fuel  at  various  boiler  efficiencies. 

Evidently,  the  actual  efficiency  of  a  given  fuel  is  here,  as  in  all  cases,  depend- 
ent upon  the  efficiency  of  the  boiler.  It  is  obvious,  however,  that  an  evap- 
oration of  15.2  pounds  of  water  from  and  at  212°  per  pound  of  best  coal 
represents  an  ideally  perfect  result,  with  100  per  cent  efficiency  of  both  fuel 
and  boiler,  unless  the  fuel  contains  sufficient  volatile  matter  to  raise  its  total 
heat  above  14,650  B.  T.  U. 

For  the  purposes  of  practical  comparison  there  is  usually  determined  the 
number  of  pounds  of  water  of  a  given  temperature  that  can  be  evaporated  into 


6o 


MECHANICAL    DRAFT. 


steam  of  a  given  pressure  by  the  combustion  of  one  pound  of  the  fuel.  This 
is  reduced  for  direct  comparison  to  the  standard  of  temperature  of  water  at 
2 1 2°,  and  steam  of  atmospheric  pressure  ;  namely,  of  a  temperature  of  2 1 2°. 
Under  these  conditions,  as  no  heat  is  expended  in  heating  the  water,  the 

Table  No.  41. — Efficiency  of  Fuels. 


o8 

C 

a 

"H  — 

EFFICIENCY  OF    BOILER. 

12 

gfa 
111 

i 

90  PER  CENT. 

80   PER  CENT. 

60  PER  CENT. 

50  PER  CENT. 

1*3 

£•13 
J=  | 

II 

||1. 

•g 

Pi 

||£j 

•3 

PL 

•o 

ilL 

-o 

IlL 

•o 

^(2 

>   Q 

£ 

0    «  CM    H  •          (j'~ 

u*—  ; 

g  «o,  « 

oV^  ' 

«  rt  £4  v 

y\_I 

>   ^Pn    - 

uVj 

1 
1 

i- 

W 

jij; 

r 

s 

i^0^ 

r 

Ip 

r 

PK 

f 

jlK; 

S  3 

r 

f 

• 

^1    N 

iij 

^1  « 

^i  « 

14,650 

15.2 

100. 

I^ 

90.0 

12.  1 

80.0 

10.6 

70.0 

9.1 

60.0 

7-6 

50.0 

14,500 

15.0 

99.0 

13-5 

89.0 

12.0 

79-2 

10.5 

69.3 

9.0 

594 

7-5 

49-5 

14.250 

i4.8 

97-3 

13-3 

87.6 

II.8 

77-8 

10.3 

68.1 

8.9 

584 

74 

48.7 

14,000 

14.5 

95.6 

I3.0 

86.0 

n.6 

76.5 

IO.I 

66.9 

8.7 

574 

7-3 

47.8 

13.750 

14.2 

93-9 

12.8 

84-5 

11.4 

75-1 

IO.O 

65-7 

8.6 

56.3 

7  i 

47-o 

13.500 

14.0 

92.2 

12.6 

83.0 

II.  2 

73-8 

9.8 

64-5 

8.4 

55-3 

7-o 

46.1 

13-250 

13-7 

90-5 

12.3 

81.5 

II.O 

72.4 

9.6 

63-4 

8-3 

54-3 

6.9 

45-3 

13,000 

'3-5 

88.8 

12.  1 

79-9 

10.8 

71.0 

94 

62.2 

8.1 

53-3 

6.7 

444 

12,750 

13.2 

87.1 

II.9 

78.4 

10.5 

69.7 

9.2 

61.0 

7-9 

52-3 

6.6 

48.6 

12,500 

12.9 

854 

n.6 

76.9 

10.3 

68.3 

9.0 

59-8 

7-7 

51.2 

6-5 

42.7 

12,250 

12.7 

837 

11.4 

75-3 

IO.I 

67.0 

8.9 

58.6 

7.6 

50.2 

6.4 

41.9 

12,000 

12.4 

82.0 

I  1.  2 

73-8 

99 

65.6 

8.7 

574 

7-5 

49-2 

6.2 

41.0 

11,750 

12.2 

80.3 

II.O 

72.3 

9-7 

64.2 

8-5 

56.2 

7-3 

48.2 

6.1 

40.2 

II,5OO 

II.9 

78.6 

10.8 

70.7 

9.6 

62.9 

8.4 

55-o 

7-1 

47.2 

6.0 

39-3 

11,250 

II-7 

769 

10.5 

69.2 

9-3 

61.5 

8.2 

53-8 

7.0 

46.1 

5-9 

38-5 

1  1  ,OOO 

II.4 

75.2 

10.2 

67.7 

9.1 

60.2 

8.0 

52.6 

6.8 

45-i 

5-7 

37-6 

10,750 

I  I.I 

73-5 

IO.O 

61.2 

8.9 

58.8 

7-8 

5'-5 

6.7 

44.1 

5-6 

36.8 

10,500 

10-9 

71.8 

9.8 

64.6 

8-7 

574 

7-6 

5°-3 

6-5 

43-  1 

54 

35-9 

IO,25O 

10.6 

70.0 

9-5 

63.0 

8-5 

56.0 

74 

49.1 

6.4 

42.0 

5-3 

35-o 

10,000 

10.4 

68-3 

9-3 

61.5 

8-3 

54-7 

7.2 

47-8 

6-3 

41.0 

5-2 

34-2 

9.750 

IO.I 

66.6 

9.1 

59-9 

8.1 

53-3 

7-i 

46.6 

6.1 

40.0 

5-1 

33-3 

9,500 

9.8 

64.9 

8.8 

584 

7-9 

51.9 

6.9 

454 

5-9 

38-9 

4.9 

32-5 

9,250 

9.6 

63.2 

8.6 

56-9 

7-7 

50.6 

6.7 

44-2 

5-7 

37-9 

4.8 

31.6 

9,000 

9-3 

61.4 

8.4 

55-3 

7-5- 

49-i 

6-5 

43-o 

5-6 

36.8 

4-7 

30-7 

8,750 

9.1 

59-7 

8.2 

53-7 

7-3 

47-8 

6.3      41-8 

54 

35-8 

4.6 

29.8 

8,500 

8.8 

58.0 

7-9 

52.2 

7-o 

46.4 

6.2 

40.6 

5-3 

34-8 

44 

29.0 

MECHANICAL    DRAFT.  61 

amount  of  heat  required  to  evaporate  one  pound  of  water  is  equal  to  the  latent 
heat  of  steam  at  atmospheric  pressure ;  that  is,  965.7  B.  T.  U.  The  evaporation 
per  pound  of  coal  or  combustible,  as  reduced  to  this  basis,  is  known  as  the  unit 
of  evaporation. 

In  illustration  of  the  method  of  calculation,  suppose  that  a  given  test  indi- 
cates that  8.73  pounds  of  water  fed  to  the  boiler  at  120°  have  been  evaporated 
into  steam  of  83.3  pounds  gauge  pressure  by  the  combustion  of  one  pound  of 
the  fuel  under  test,  without  correction  for  moisture  in  steam  and  fuel.  The 
absolute  steam  pressure  is  83.3-]-  14.7  =98  pounds,  and  per  Table  No.  6  the 
total  amount  of  heat  contained  in  one  pound  of  steam  of  this  pressure  is 
1,213.40  B.  T.  U. ;  while  per  Table  No.  3  the  total  heat  of  the  water  of  120°  tem- 
perature is  120.149  B.  T.  U.  Evidently,  then,  the  amount  of  heat  which  was 
imparted  to  one  pound  of  water  at  120°  in  order  to  convert  it  into  steam  of  83.3 
pounds  gauge  pressure  was  1,213.40 — 120.149  =  1,093.251  B.  T.  U.  As  the 
latent  heat  of  steam  at  atmospheric  pressure  is  965.7  B.  T.  U.,  the  evaporation 
of  8.73  pounds  of  water  under  the  stated  conditions  is  equivalent  to  the  evap- 
oration of  — 

1'°93-2Sl   X  8.73  =  9.87  pounds, 

965-7 
from  and  at  212°. 

For  ultimate  comparison  of  fuels  the  results  should  be  corrected  for  moisture 
in  the  steam  and  in  the  fuel.  If,  under  the  conditions  of  the  test  just  used  for 
illustration,  the  steam  had  contained  1.2  per  cent  and  the  coal  3.5  per  cent  of 
moisture,  the  method  of  correction  would  be  as  follows :  — 

The  actual  proportion  of  dry  steam  would  be  100 — 1.2  =  98.8  per  cent,  and 
that  of  dry  coal,  100  —  3.5  =96.5  per  cent.  The  amount  of  water  evaporated 
into  dry  steam  from  and  at  212°  per  pound  of  fuel  would,  therefore,  be 
9.87  x  0.988  =  9.75  pounds  ;  and  the  evaporation  of  dry  steam  per  pound  of  dry 
fuel  would  be  9. 75  -=-0.965  =  10.10  pounds;  or,  combined  in  one  calculation, — 

9.87  x  0.988  _10.         unds. 

0.965 

To  ascertain  the  equivalent  evaporation  per  pound  of  combustible  in  the  fuel, 
the  proportion  of  ash  must  be  ascertained  by  careful  weighing  and  deducted 
from  the  total  fuel  burned.  If  the  ash  in  the  fuel  from  which  the  preceding 
results  were  obtained  had  amounted  to  6.4  per  cent,  the  evaporation  of  water 
from  and  at  212°  into  dry  steam  would  have  been  — 

TO  IO 

_  =  10.79  pounds, 
i.oo- — 0.064 


62  MECHANICAL    DRAFT, 

Table  No.  42,  calculated  by  the  method  previously  explained,  presents  a  series 
of  factors  by  means  of  any  one  of  which,  corresponding  to  the  given  tempera- 
ture of  feed  water  and  pressure  of  steam,  the  evaporative  result  obtained  may 
be  transformed  into  the  equivalent  evaporation  from  and  at  212°.  Thus,  taking 
the  conditions  of  83.3  pounds  and  120°  temperature  already  given,  the  factor 
(ascertained  by  interpolation)  is  1.132,  which,  multiplied  by  8.73,  gives  9-881 
pounds  of  water  evaporated,  from  and  at  212°.  Evidently,  this  table  maybe 
used  in  a  converse  manner  to  determine  what  conditions  of  feed  temperature  and 
steam  pressure  maybe  equivalent  to  a  stated  evaporation  from  and  at  212°. 
Thus,  for  instance,  an  evaporation  of  9.87  pounds  from  and  at  212°  is  equiva- 
lent to  9.87  -f-  1.149  =8.59  pounds  from  water  at  100°  into  steam  at  70  pounds 
gauge  pressure. 

Relative  Efficiency  of  Various  Coals.  —  Although  the  preceding  applies  to  all 
classes  of  fuel,  the  greatest  interest  centres  in  the  practical  calorific  value  of 
various  kinds  of  coal ;  for  upon  this  fuel,  above  all  others,  is  general  steam-boiler 
practice  most  dependent.  It  must  have  already  become  evident  that  the 
apparent  efficiency  of  the  coal  and  of  the  boiler  in  connection  with  which  it  is 
burned  are  interdependent.  Increased  calorific  value  on  the  part  of  the  coal 
insures  an  increase  in  the  output  of  the  boiler ;  while  an  improvement  in  the 
proportions  of  the  boiler,  its  appurtenances  or  its  method  of  operation,  whereby 
its  steaming  power  is  increased  per  pound  of  coal,  likewise'  raises  the  practical 
efficiency  of  the  coal. 

In  general  it  may  be  stated  that  any  furnace  is  well  adapted  to  the  combus- 
tion of  anthracite  and  semi-bituminous  coals  containing  less  than  20  per  cent 
of  volatile  matter.  For  coals  containing  between  20  and  40  per  cent,  a  plain 
grate-bar  furnace  with  firebrick  arch  thrown  over  it  is  desirable,  because  of  its 
ability  to  keep  the  furnace  chamber  hot.  For  coals  which  contain  over  40  per 
cent  of  volatile  matter,  a  furnace  is  desirable  which  is  surrounded  by  firebrick, 
with  a  large  combustion  chamber  a»nd  special  appliances  for  introducing  very  hot 
air  to  the  gases  distilled  from  the  coal.  A  separate  gas  producer  and  combus- 
tion chamber,  arranged  for  heating  both  air  and  gases  before  they  unite,  serves 
the  same  purpose. 

The  efficiency  of  a  given  coal  is  dependent,  not  only  on  its  chemical  composi- 
tion and  theoretical  heat  value,  but  to  a  great  degree  upon  the  percentage  of 
ash  and  moisture  which  it  contains,  and  upon  the  size  of  the  respective  pieces 
or  particles,  both  absolutely  and  relatively  to  each  other. 


i  The  slight  difference  between  this  and  the  calculated  result  is  due  to  the  fact  that  the 
numbers  in  the  table  are  not  carried  out  to  more  decimal  places. 


MECHANICAL    DRAFT. 
Table  No.  42.  —  Factors  of  Evaporation. 


PRESSURE  IN  POUNDS  PER  SQUARE  INCH  ABOVE  THE  ATMOSPHERE. 


Ip" 

° 

20 

40 

5» 

60 

70 

80 

90 

100 

120 

140 

1  60 

320 

.187 

.201 

1.211 

I.2I4 

I.2I7 

1.219 

.222 

1.224 

1.227 

I.23I 

1-234 

•237 

35 

.184 

.198 

1.208 

1.  211 

I.2I4 

.216 

.2I9 

1.  221 

1.224 

1.228 

1.231 

•234 

40 

.179 

•193 

1.203 

1.  206 

1.209 

.211 

.214 

1.216 

I.2I9 

1.223 

1.226 

.229 

45 

.173 

.187 

I.I97 

1.200 

1.203 

.205 

.208 

I.2IO 

I.2I3 

I.2I7 

i.  220 

.223 

50 

.168 

.182 

I.I92 

.I95 

I.I98 

.200 

.203 

1.205 

1.208 

1.  212 

1.215 

.218 

55 

.163 

.177 

I.l87 

.I90 

I-I93 

•195 

.198 

1.200 

1.203 

1.207 

1.  210 

.213 

60 

.158 

.172 

I.I82 

.185 

I.I88 

.100 

•'93 

I-I95 

I.I08 

I.2O2 

1.205 

.208 

65 

•153 

.167 

I.I77 

.ISO 

1.183 

.185 

.188 

I.I9O 

I  -.193 

I.I97 

1.200 

.203 

70 

.148 

.162 

I.I72 

•»75 

I.I78 

.ISO 

.I83 

I.l85 

I.I88 

I.I92 

LI95 

.I98 

75 

-143 

•T57 

I.l67 

.170 

I-I73 

•«75 

.178 

I.ISO 

I.I83 

1.187 

I.IOO 

•'93 

80 

•X37 

.151 

U6I 

.164 

1.167 

.169 

.172 

I.I74 

I.I77 

1.181 

1.184 

.187 

85 

.132 

.146 

I.I56 

•159 

I.I62 

.164 

.167 

I.l69 

I.I72 

1.176 

1.179 

.182 

90 

.127 

.141 

I.I5I 

-154 

I.I57 

•159 

.162 

1.164 

I.l67 

1.171 

1.174 

•177 

95 

.122 

.136 

1.146 

.149 

I.I52 

-154 

•'57 

I.I59 

I.I62 

1.166 

1.169 

.172 

1  00 

.117 

•131 

I.I4I 

.144 

I.I47 

.149 

.152 

I-I54 

I.I57 

1.161 

1.164 

.167 

105 

.III 

.125 

«-I35 

•139 

I.I4I 

•M3 

.146 

I.I48 

I.ISI 

'•'55 

1-158 

.l6l 

IIO 

.106 

.120 

I.I3O 

•133 

1.136 

.138 

.141 

i-M3 

I.I46 

1.150 

I.I53 

.156 

"5 

.101 

.115 

I.I25 

.128 

I.I3I 

•133 

.136 

1.138 

I.I4I 

i-i45 

1.148 

.I5I 

120 

1.096 

.110 

I.I2O 

.123 

I.I26 

.128 

.131 

I-I33 

I.I36 

1.140 

i-M3 

-I46 

I25 

I.Ogi 

.105 

1.115 

.118 

1.  121 

.123 

.126 

1.128 

I.I3I 

l^35 

1.138 

.141 

IjO 

1.085 

1.099 

I.IO9 

.112 

I.II5 

.117 

.120 

1.  122 

I.I25 

1.129 

1.132 

•'35 

135 

I.OSO 

1.094 

I.I04 

.107 

I.  IIO 

.112 

•MS 

I.II7 

1.  1  2O 

1.124 

1.127 

.130 

140 

'•075 

1.089 

1.099 

.IO2 

I.I05 

.107 

.IIO 

1.  112 

I.II5 

1.119 

1.  122 

•I25 

M5 

I.O70 

1.084 

1.094 

1.097 

1.  100 

.102 

.105 

I.IO7 

I.  IIO 

1.114 

I.II7 

.120 

150 

1.065 

1.079 

I.089 

I.O92 

1.095 

1.097 

.100 

I.IO2 

I.IO5 

1.109 

1.  112 

•"5 

'55 

1.059 

1-073 

1.083 

1.  086 

1.089 

I.Ogi 

1.094 

1.096 

1.099 

1.103 

I.I06 

.109 

160 

1.054 

1.068 

1.078 

I.OSI 

1.084 

1.086 

1.089 

I.09I 

1.094 

1.098 

I.IOI 

.104 

165 

1.049 

1.063 

1-073 

1.076 

1.079 

1.  08  1 

1.084 

1.  086 

I.089 

1.093 

1.096 

1.099 

170 

1.044 

1.058 

1.068 

I.07I 

1.074 

1.076 

1.079 

I.oSl 

1.084 

i.  088 

I.09I 

1.094 

1.039 

I-053 

1.063 

1.  066 

1.069 

I.07I 

1.074 

1.076 

1.079 

1.083 

1.086 

1.089 

1  80 

'•033 

1.047 

1-057 

I.  OOO 

1.063 

1.065 

1.068 

I.O7O 

1-073 

1.077 

1.  080 

1.083 

185 

1.028 

1.042 

I.O52 

1.055 

1.058 

1.  000 

1.063 

1.065 

1.  068 

1.072 

1-075 

10.78 

190 

1.023 

I-°37 

1.047 

I.O5O 

I-°53 

LOSS 

1.058 

I.  OOO 

1.063 

1.067 

I.O7O 

1-073 

»95 

I.OI8 

1.032 

I.O42 

1.045 

1.048 

1.050 

1-053 

'•055 

1.058 

1.062 

1.065 

1.068 

200 

I.OI3 

1.027 

1-037 

1.040 

1-043 

1.045 

1.048 

I.O50 

'•053- 

1.057 

I.  OOO 

1.063 

205 

1.008 

1.022 

1.032 

1-035 

1.038 

1.040 

1.043 

1.045 

1.048 

1.052 

'•055 

1.058 

210 

I.OO4 

I.OI7 

1.027 

I.O30 

1-033 

1.035 

1.038 

I.O4O 

1.043 

1.047 

1.050 

1-053 

212 

I.O02 

1.  000 

















64  MECHANICAL    DRAFT. 

An  efficiency  of  100  per  cent  on  the  part  of  either  the  coal  or  the  boiler  is 
an  absolute  impossibility  because  of  certain  losses  which  are  incident  to  the 
combustion  of  the  coal  and  the  operation  of  the  boiler.  Of  these  losses  some 
are  inevitable,  while  others  may  be  diminished  or  avoided. 

The  unavoidable  losses  are  :  — 

First.  The  heat  loss  by  converting  into  steam  the  water  contained  in  the 
coal,  and  in  the  air  used  in  burning  it,  as  well  as  that  formed  by  the  burning  of 
the  hydrogen  and  the  heating  of  the  steam  thus  formed  to  the  temperature  at 
which  the  gases  leave  the  stack. 

Second.  The  heat  necessary  to  raise  to  the  stack  temperature  the  carbonic 
acid  gas  formed  by  burning  the  carbon,  the  nitrogen  originally  present  in  the 
air  from  which  the  oxygen  has  been  taken  to  form  carbonic  acid,  the  sulphurous 
acid  and  the  excess  of  air  which  is  supplied  to  secure  perfect  combustion. 
There  is  also  a  loss  through  heat  in  the  hot  ashes  removed  from  the  ash  pit, 
as  well  as  through  the  unconsumed  carbon  remaining  in  the  ash.  A  further 
loss  occurs  through  radiation  from  the  boilers  and  walls,  which  can,  by  careful 
construction  and  covering,  be  greatly  reduced  but  never  eliminated. 

The  losses  which  are  more  or  less  avoidable  are :  — 

First.  Those  due  to  incomplete  combustion,  as  evidenced  in  the  presence  of 
smoke  and  carbonic  oxide  in  the  flue  gases  and  in  unconsumed  coal  in  the 
ashes.  This  latter  loss  is  due  to  the  original  small  size  or  the  subsequent 
decrepitation  of  the  coal,  which  results  in  the  dropping  of  more  or  less  of  it 
through  the  grates  without  being  consumed.  In  addition  a  small  amount  of 
hydrogen  or  marsh  gas  may  pass  out  with  the  gases. 

Second.  Loss  from  excess  of  air,  due  to  the  fact  that  to  secure  practically 
perfect  combustion  air  must  be  supplied  considerably  in  excess  of  the  theoreti- 
cal quantity  chemically  required  for  combustion.  This  loss  is  twofold,  being 
dependent  upon  the  quantity  of  unused  oxygen  and  associated  nitrogen  and 
upon  the  moisture  in  the  air. 

Third.  The  loss  resulting  from  too  high  temperature  of  the  gases  leaving  the 
boiler.  This  loss,  except  in  so  far  as  it  is  influenced  by  the  air  supply  and  the 
rate  of  combustion,  is  dependent  upon  the  design  of  the  boiler  and  its  appurte- 
nances, and,  therefore,  is  not  chargeable  to  the  character  of  the  fuel.  It  is  one 
of  the  most  important  factors  in  fuel  efficiency. 

Fourth.  Loss  of  heat  by  removing  ashes  at  too  high  a  temperature.  This,  by 
care,  may  be  reduced  but  not  entirely  avoided. 

Fifth.  Loss  by  radiation.  This  may  be  reduced  by  increasing  the  thickness 
of  walls  and  covering  all  exposed  portions  of  the  boiler.  But  from  a  practical 
standpoint  it  can  never  be  entirely  avoided. 


MECHANICAL    DRAFT. 


The  influence  of  these  various  sources  of  loss  upon  the  efficiency  of  fuels 
and  boilers  will  be  considered  in  succeeding  pages.  Independent  of  such  con- 
sideration the  relative  efficiency  of  various  coals,  as  indicated  by  comparative 
tests,  may,  however,  be  here  introduced.  It  is  already  evident  that,  for  the  pur- 
poses of  strict  comparison  of  evaporative  powers,  coals  should  be  tested  under 
identical  conditions.  What  is  more,  all  ordinary  grades  of  coal  should  be 
tested  under  such  a  variety  of  boilers  and  rates  of  combustion,  air  supply  and 
draft  that  they  may  be  intelligently  compared  one  with  the  other  under  all  ordi- 
nary conditions.  Such  extensive  and  strictly  comparable  results  do  not,  as  yet, 
exist,  although  already  much  careful  work  has  been  and  is  now  being  done  to 
furnish  such  a  basis  of  comparison. 

At  the  present  time  it  is  only  possible  to  compare  with  each  other  the  results 
in  certain  groups  of  tests,  but  only  to  a  limited  extent  to  correspondingly  com- 
pare the  results  in  one  group  with  those  in  another.  By  such  comparisons  as 
are  allowable,  it  is  possible  to  approximate  with  reasonable  accuracy  to  the 
relative  values  of  the  coals  under  consideration. 

A  series  of  such  results,  compiled  from  the  reports  of  numerous  tests  by 
Mr.  George  H.  Barrus,1  is  presented  in  Table  No.  43.  In  harmony  with  the 

Table  No.  43.  —  Comparative  Evaporative  Efficiency  of  Various  Coals. 


KIND  OF  COAL. 

Water  Evaporated 
from  and  at  212°  by  One 
Pound  of  Dry  Coal. 

Relative  Efficiency 
in  per  cent. 
Cumberland  =  100. 

Cumberland, 

. 
11.04 

100 

Anthracite,  Broken, 

9-79 

89 

Anthracite,  Chestnut, 

9.40 

85 

Two  parts  Pea  and  Dust  and  one  part  Cumberland,     |                 9-3^ 

85 

Two  parts  Pea  and  Dust  and  one  part  Culm, 

9.01 

82 

Anthracite,  Pea, 

8.86 

80 

Nova  Scotia  Culm, 

8.42 

76 

results  obtained  in  a  large  number  of  tests,  the  evaporation  per  pound  of  com- 
bustible in  anthracite  broken  coal  has  been  taken,  as  a  standard  of  comparison, 
in  round  numbers  at  n.o  pounds  from  and  at  212°.  With  an  average  of  n  per 
cent  of  ash  this  is  equivalent  to  9.79  pounds  of  water  evaporated  from  and  at 
212°  by  one  pound  of  coal. 

Other   comparisons    of    efficiency  between    different    kinds    of   fuel    may  be 
drawn  from  the  various  reports  of  tests  upon  succeeding  pages. 


1  Boiler  Tests.     George  H. 


Boston,  1891. 


66 


MECHANICAL   DRAFT. 


Influence  of  Ash.  —  Beyond  its  indication  of  the  relative  presence  of  incom- 
bustible matter,  the  influence  of  ash  upon  the  thermal  efficiency  of  coal  is  three- 
fold. Its  presence  is  the  measure  of  the  loss  through  the  amount  of  unconsumed 
carbon  which  it  contains,  through  the  heat  lost  when  the  ash  is  removed  in  a 
heated  condition,  and  through  the  influence  of  the  ash  in  clogging  the  fire  and 
preventing  free  combustion.  From  a  commercial  standpoint  its  presence, 
furthermore,  proportionately  increases  the  original  cost  of  freight  and  handling 
per  heat  unit  derived,  as  well  as  the  subsequent  expense  incident  to  its  own 
removal  and  transportation  to  a  proper  dumping-place,  —  a  fact  which  demands 
careful  consideration. 

The  percentage  of  carbon,  either  in  the  form  of  cinder  or  decrepitated  coal, 
which  eventually  forms  a  part  of  the  ash,  can  be  somewhat  reduced  by  skilful 
manipulation  of  the  fire.  But  as  the  greatest  saving  of  such  carbon  lies  in 
slower  and  gentler  firing  and  in  admitting  more  air,  the  efficiency,  as  a  whole,  is 
liable  to  be  lowered  rather  than  raised  if  the  attempt  to  economize  is  carried  to 
an  extreme.  The  carbon  ordinarily  present  in  ash  varies  greatly,  but  may  be 
broadly  stated  to  range  between  10  and  60  per  cent.  The  loss  by  carbon  in  the 
ash  is,  therefore,  dependent  upon  the  percentage  of  ash  in  the  coal,  as  is  shown 
in  Table  No.  44. 

Table  No.  44.  —  Loss  of  Fixed  Carbon  on  Account  of  Carbon  in  Ash. 


Per  cent  of  1 

Carbon  in  Ash. 

PER  CENT  OF  ASH  IN  COAL  FIRED. 

9 

per  cent. 

per  cent. 

per  cent. 

12 

per  cent. 

13 

per  cent. 

14 

per  cent. 

15 

per  cent. 

16 
per  cent. 

'7 
per  cent. 

18 
per  cent. 

75 

3I-76 

35-70 

39-73 

43.88 

48.14 

52-47 

56.94 

61.51 

66.23 

71.05 

70 

24.70 

27-75 

30.89 

34-n 

37-43 

40.80 

44.27 

47-83 

5I-51 

55-26 

65 

19.65 

22.09 

24-59 

27.16 

29.79 

32.48 

35-23 

37-07 

41.00 

43-98 

60 

15.87 

17.84 

19.86 

21.93 

24.07 

26.23 

28.47 

30.76 

33-" 

35-52 

55 

12.94 

M-54 

16.18 

17.87 

19.60 

21-37 

23.19 

24.05 

26.98 

-28.94 

5° 

10.59 

II.90 

13.24 

14.63 

16.04 

17.49 

18.98 

20.50 

22.08 

23.68 

45 

8.66 

9.76 

10.83 

11.97 

13.12 

i4-3i 

!5-52 

16.77 

1  8.06 

19.38 

40 

7-05 

7.92 

8.82 

9-74 

10.69 

11.66 

12.64 

13.66 

14.72 

15-79 

35 

5-69 

6.40 

7.12 

7.87 

8.63 

9.40 

10.21 

11.04 

u.88 

12-75 

3° 

4-53 

5-10 

5-66 

6.27 

6.87 

7-43 

8.12 

8.78 

9-45 

10.14 

25 

3-52 

3-96 

4.41 

4.96 

5-34 

5-82 

6.3I 

6.83 

7-35 

7.89 

20 

2.64 

2.97 

3-3° 

3-65 

4.02 

4-36 

4-74 

5-l3 

S-S2 

5-92 

15 

1.86 

2.09 

2-33 

2-57 

2.82 

3-09 

3-35 

3.61 

3-89 

4.18 

10 

1.17 

1.32 

i-47 

1.62 

1.77 

i-93 

2.10 

2.27 

2-45 

2.62 

5 

0.56 

0.63 

0.69 

0.76 

0.84 

0.91 

0-99 

1.07 

1.13 

1.24 

i 

O.IO 

0.12 

0.13 

0.14 

o.i  6 

0.17 

0.18 

0.20 

0.22 

0.24 

MECHANICAL   DRAFT.  67 

It  is  obvious  that  the  greater  the  amount  of  hot  ashes  resulting  from  the  com- 
bustion of  a  given  amount  of  coal,  the  greater  will  be  the  loss  of  heat  when 
these  ashes  are  removed.  As  a  means  of  clogging  the  grates,  preventing  free 
combustion  and  necessitating  extra  work  on  the  part  of  the  fireman,  with  a 
resulting  excess  of  air  while  the  fire  doors  are  open,  a  large  amount  of  ash  in 
the  fuel  exerts  a  very  important  influence. 

Influence  of  Moisture  in  Coal.  —  Moisture  in  coal  is  an  exceedingly  variable 
quantity,  depending  upon  the  character  of  the  coal,  its  temperature  and  its  pre- 
vious exposure  to  the  atmosphere.  Under  ordinary  conditions  its  percentage 
varies  from  i  to  5  per  cent.  Whatever  its  amount,  it  must  all  be  raised  to  2 1 2°, 
evaporated  into  steam  and  the  steam  raised  to  the  temperature  of  the  escaping 
gases.  It,  therefore,  has  an  important  influence  upon  the  theoretical  heat  value 
of  a  given  coal.  Thus,  if  one  coal  was  composed  of  80  per  cent  carbon,  15  per 
cent  ash  and  5  per  cent  water,  and  another  consisted  of  the  same  proportion  of 
carbon,  with  5  per  cent  ash  and  15  per  cent  water,  the  theoretical  calorific  value 
—  viz.,  11,720  B.  T.  U. —  would  be  the  same,  being  directly  dependent  upon  the 
amount  of  carbon.  But  in  the  first  case  the  available  heat  (neglecting  losses 
not  due  to  water)  would  be  10,600  B.  T.  U.,  while  in  the  second  it  would  be 
10,488  B.  T.  U.,  if  the  waste  gases  were  assumed  to  escape  at  500°. 

Influence  of  Size  of  Coal.  —  A  still  further,  and  under  certain  conditions  a 
very  important,  influence  upon  the  efficiency  of  coals,  particularly  the  anthra- 
cites, is  exerted  by  the  size  of  their  respective  pieces  or  particles.  For  the  freest 
burning  they  should  be  as  nearly  of  a  size  as  possible ;  hence,  the  screening 
process  at  the  mines  and  their  sale  in  stated  sizes.  In  the  smaller  sizes  of 
anthracite,  consisting  of  culm  and  screenings,  or  slack,  the  inherent  dust  and 
minute  particles  render  them  difficult  coals  to  burn  unless  mixed  with  a  certain 
proportion  of  bituminous  coal  and  burned  upon  special  grates,  with  an  intensity 
of  draft  which  can  only  be  economically  produced  by  mechanical  means.  This 
feature  becomes  more  pronounced  as  the  coal  becomes  finer,  and  makes  legiti- 
mate comparison  with  other  coals  a  somewhat  difficult  matter. 

Broadly  stated,  the  requisites  to  success  in  the  combustion  of  small  anthra- 
cites are,  as  expressed  and  explained  by  Mr.  Geo.  H.  Ward,1  "first,  draft;  and, 
second,  manipulation  of  the  fires.  Of  course  no  fire  will  burn  without  draft, 
and  the  greater  the  amount  of  fire  in  a  given  space  the  stronger  the  draft  must: 
be  to  properly  consume  the  fuel.  With  the  larger  sizes  of  fuel  this  question  of 
draft  is  less  prominent,  but  when  we  come  to  burn  the  smaller  sizes  the  draft  is 


'The  Economy   of  Small   Anthracite   Coals.     Geo.  H.  Ward.     A  paper  read  before   the 
Engineers'  Club  of  Brooklyn,  N.  Y.     1892. 


68 


MECHANICAL   DRAFT. 


of  the  utmost  importance.  The  coal  will  pack  on  the  grate,  and,  owing  to  the 
way  the  pieces  will  arrange  themselves  one  to  another,  it  will  be  impossible  to 
get  sufficient  air  through  the  bed  of  fire  unless  the  draft  is  strong  enough  to 
displace  the  smaller  particles.  .  .  .  Another  and  perhaps  a  better  reason  is 
that  the  proportion  of  ash  is  somewhat  greater  with  the  smaller  coals,  and  as 
the  fire  is  but  a  thin  crust  on  top  of  this,  it  follows  that  a  somewhat  stronger 
draft  will  be  required  to  get  the  necessary  volume  of  air  through  the  bed  of 
ashes  and  the  closely  packed  crust  of  the  fire  on  top." 

The  matter  of  burning  the  smaller  sizes  of  refuse  coal  will  be  still  further  con- 
sidered from  a  commercial  standpoint,  but  the  conditions  which  control  the 
successful  combustion  of  such  fuel  may  be  here  discussed.  While  the  terms 
"  screenings  "  and  "  slack  "  are  generally  applied  to  the  refuse  of  local  coal  yards, 
the  term  "  culm  "  is  restricted  in  its  application  to  the  refuse  or  waste  from  the 
anthracite  coal  mines.  Originally  applied  to  any  mixture  below  pea  coal,  this 
term  has  become  more  restricted  in  its  meaning,  as  the  smaller  sizes  have 
been  removed  in  the  ordinary  process  of  screening,  until  at  the  present  time  it 
is  in  some  localities  applied  to  that  which  cannot  be  sold  as  buckwheat,  but 
which  has  had  a  part  of  the  dust  washed  out.  It  is,  therefore,  evident  that  in 
any  discussion  of  the  use  of  a.  refuse  fuel  more  than  its  name  is  necessary  to 
determine  its  exact  character. 

Although  there  is  considerable  variety  in  the  size  classification  of  coal  at 
different  mines,  the  generally  accepted  dimensions,  as  determined  by  the  limit- 
ing diameters  of  the  perforations  in  the  screens,  are  about  as  presented  in  Table 
No.  45. 

Table  No.  45.  — Sizes  of  Coal. 


DESIGNATION. 


Diameter  of  Diameter  of 

Perforation  over  which  !      Perforation  through 
Coal  will  pass.  j    which  Coal  will  pass. 


Dust 

3-32  inch. 

No.  3 

Buckwheat   .         .         .         .         .         . 

3-32  inch.                      3-16     " 

Bird's-eye        .         .         .         .         .                  . 

1-8        "                         5-16     " 

No.  2 

Buckwheat,  or  Rice 

3-1  6     '<                         3-8       « 

No.  i 

Buckwheat  .....'. 

3-8        "                         9-16     " 

Pea 

.         .         .         .         .  •  •    ''. 

9-16     "                         7-8 

Chestnut         

s-s     «             iys     » 

Small 

Stove    .         .        ... 

I            "                     1^        " 

Large  Stove    

*X      "                2X 

Egg 

2^             "                                2% 

Broken            .         . 

2%             "                                4 

Stearr 

boat      

4             "                       7             " 

MECHANICAL    DRAFT,  69 

The  important  factors  in  the  successful  burning  of  small  anthracite  coals 
are  :  — 

First.   Mechanical  draft,  preferably  applied  beneath  the  grate. 

Second.  Large  grate  area. 

Third.  Grate  constructed  with  a  practically  plain  surface  to  prevent  lodge- 
ment of  coal. 

Fourth.  Grate  of  proper  design  for  ready  removal  of  ash  and  clinker. 

Fifth.  Air  spaces  not  over  Jff  to  T3F  inch  wide,  except  for  No.  i  buckwheat  or 
bituminous  slack,  for  which  they  may  be  %  inch  wide. 

Sixth.  Arrangements  to  allow  of  the  feeding  and  cleaning  of  fires  without 
excessive  opening  of  doors  or  dropping  of  ashes  into  ashpit. 

Seventh.  Thin  fires  and  frequent  and  careful  firing.  The  thickness  of  the  bed 
should  diminish  with  the  rate  of  combustion. 

Eighth.  Reduction  of  draft  above  the  fire  as  the  rate  of  combustion  decreases. 
The  fine  character  of  the  material,  as  a  result  of  which  its  particles  pack  closely 
together  with  but  small  intersticial  spaces,  makes  strong  draft  imperative  in 
order  to  secure  the  passage  of  the  proper  amount  of  air  through  the  bed  of 
fuel.  That  mechanical  draft  is  the  necessary  and  most  economical  means  of 
securing  this  result  is  the  verdict  of  all  who  have  had  experience  in  the  burning 
of  such  fuel.  Owing  to  the  usually  somewhat  complicated  grate  or  feeding 
arrangements  for  the  burning  of  small  anthracites,  forced  draft  is  usually 
applied  beneath  the  grates,  but  may  under  certain  conditions  be  favorably 
assisted  by  supplementary  induced  draft.  The  careful  and  extensive  experi- 
ments of  Mr.  Eckley  B.  Coxe,  upon  the  burning  of  small  anthracite  coals,  have 
served  to  throw  considerable  light  upon  the  features  and  requirements  of  such 
undertakings.  He  concludes1  "  that  it  is  possible  that  the  best  results  in  burn- 
ing these  small  coals  may  be  obtained  by  using  a  blower  under  the  grate  and 
a  suction  apparatus  in  the  stack." 

A  large  grate  area  permits  of  the  carrying  of  thinner  fires  with  a  given  total 
consumption  of  coal,  while  the  small  spaces  through  the  grate  are  necessary  to 
prevent  too  great  loss  by  the  dropping  of  fine  coal  through  them.  The  neces- 
sity of  such  grates  is  shown  in  the  report  of  the  commission2  of  which  Mr. 
Coxe  was  a  member  and  for  whom  these  experiments  were  instituted:  "A 
careful  study  of  the  burning  of  culm,  —  that  is,  the  burning  of  coals  with  more 
or  less  dust  in  them,  —  in  these  and  other  experiments,  seemed  to  show  that  in 


1  Furnace  for  Burning  Small  Anthracite  Coals.     Eckley  B.  Coxe.     Trans.  Am.  Inst.  Mining 
Engineers,  Vol.  XXII.     1894. 

2  Commonwealth  of   Pennsylvania.     Report  of   Commission  Appointed  to  Investigate   the 
Waste  of  Coal  Mining,  with  the  View  to  the  Utilizing  the  Waste.     May,  1893. 


7o  MECHANICAL   DRAFT. 

almost  all  cases  it  is  accompanied  by  a  very  high  percentage  of  carbon  in  the 
ash,  which  analysis  showed,  in  some  cases,  reached  58  per  cent.  Unless  special 
precautions  are  taken  to  prevent  it,  a  large  portion  of  the  fine  coal  runs  down 
through  the  grate.  When  culm  gets  red  hot  it  acts  almost  like  dry  sand  and 
works  its  way  into  the  ashpit,  thus  increasing  largely  the  percentage  of  carbon. 
When  coal  has  to  be  transported  any  distance,  the  value  of  the  culm  at  the 
mines  being  very  small,  it  is  probable,  from  the  investigations  made,  that  it 
would  be  cheaper  to  remove  the  dust  and  transport  only  the  larger  coal." 

It  appears  that  the  combustion  of  small  anthracites  is  more  perfect  when  the 
coal  remains  undisturbed,  or  as  nearly  as  possible  in  the  condition  in  which  it 
was  put  upon  the  fire,  instead  of  being  turned  over  so  that  the  partially  con- 
sumed and  unconsumed  coal  are  mixed  together.  For  such  fuel  the  travelling 
grate  is  particularly  adapted,  as  it  leaves  the  fuel  undisturbed,  and  makes  pos- 
sible a  gradation  of  the  draft  to  meet  the  varying  conditions  incident  to  the 
progress  of  the  grate  and  the  combustion  of  the  fuel  upon  it. 

Whatever  the  size  of  these  smaller  grades  of  fuel,  certain  special  furnace 
arrangements  are  necessary.  The  smaller  they  are  the  more  extensive  and 
expensive  the  appliances ;  and  for  each  special  arrangements  are  necessary.  It 
is,  therefore,  undesirable  and  positively  uneconomical  to  mix  the  sizes.  Thus  a 
mixture  of  dust  with  pea  or  chestnut  coal,  while  burning  more  freely  because  of 
the  easier  passage  of  air  through  it,  will  not  give  so  good  evaporative  results  as 
an  intermediate  size  containing  less  of  the  larger  and  the  smaller  pieces  or  dust ; 
that  is,  having  all  of  its  pieces  nearer  a  size.  This  difference  is  due  to  the  fact 
that  in  the  mixture  the  fine  coal  is  completely  consumed  before  the  larger  pieces. 

The  advantage  of  mixing  a  slight  amount  of  bituminous  coal  with  the  smaller 
anthracites  is  well  known.  The  somewhat  glutinous  character  of  the  former, 
when  burning,  serves  to  make  a  more  coherent  mass  of  the  entire  body  of  fuel, 
thereby  preventing  the  dust  from  being  blown  through  the  flues  or  dropped 
through  the  grates,  while  keeping  the  bed  more  open  and  thereby  increasing  the 
rate  of  combustion.  Culm  and  rice  coal  thus  fired  give  results  in  total  evapora- 
tion which  cannot  be  reached  by  the  same  anthracites  alone. 

The  experiments  of  Mr.  Coxe  have  demonstrated  that  "  the  temperature  de- 
veloped by  the  burning  of  the  smaller  coals  decreases  with  the  size  of  the  coal ; 
this  naturally  involves  a  larger  heating  surface  in  the  boiler  in  order  to  develop 
the  same  number  of  horse-powers ;  that  is  to  say,  if  you  are  burning  pea  coal, 
and  obtaining  one  horse-power  for  every  nine  square  feet  of  heating  surface, 
you  would  probably  require  from  20  to  25  per  cent  more  heating  surface  if  you 
are  using  No.  3  buckwheat ;  although  you  may  be  evaporating  practically  the 
same  amount  of  water  per  pound  of  coal." 


MECHANICAL   DRAFT. 


Table  No.  46  gives  results  of  tests1  of  small  anthracites  by  Mr.  Coxe. 
Table  No.  46.  —  Results  of  Tests  of  Pea  and  Buckwheat  Coals. 

No.  OF  TEST  AND  KIND  OF  FUBI» 


ITEMS. 

Oneida 
Pea  Coal. 

2 

Oneida 

No.  i 
Buckwh't. 

Oneida 
No.  2 
Buckwh't. 

Oneida 
No".  3 
Buckwh't. 

Eckley 
No.  3 
Buckwh't. 

Pounds  of  water  evaporated  per  pound  of  dry  \  j 
coal,  actual  conditions,                                        j  i     •'  * 

6.62 

7-17 

7-21 

7.36 

Pounds  of  water  evaporated  per  pound  of  dry  J 
coal  from  and  at  2120,                                         j 

8.56 

7-94 

8.60 

8.65 

8.74 

Pounds  of  water  evaporated  per  pound  of  com-  \ 
bustible  from  and  at  2120,                                  J 

1  0.06 

10-57 

II.  12 

II.  IO 

Pounds  of  water  evaporated  from  and  at  2120) 
per  square  foot  of  heating  surface,                   J  1     *v 

3.21 

3-13 

3-13 

3-06 

Pounds  of  coal  per  square  foot  of  grate  per  hour,    1  3.63 

13-58 

11.40 

"•34 

9-44 

Average 

temperature  of  escaping  gases, 

549° 

543° 

498° 

5°3° 

372° 

Moisture  in  steam,  per  cent, 

2.2 

2.0 

I.9 

1.9 



Moisture  in  coal  as  fired,  per  cent, 

2.63 

4.06 

8.62 

6-53 

4-93 

Per  cent  of  ash, 

15.60 

20.  1  0 

18.71 

22.27 

2i-3 

Per  cent 

of  carbon  in  ash, 

I5-85 

12-35 

9-33 

31.90 

29.63 

Average  pressure  of  blast  in  inches  of  water  in  | 
entrance  chamber,                                                j 

o-375 

0.5 

0.625 

1.04 

1.125 

f  Water  at  2250, 

2.15 

2.00 

2.10 

2.05 

2.50 

Analy-  j  Volatile  combustible  matter, 

5.10 

4.90 

5-45 

5.42 

5-oo 

sis  of  -< 

Ash, 

12-55 

17-35 

I5-50 

12.90 

13-97 

coal. 

Carbon,  fixed, 

80.20 

75-75 

76.95 

79-63 

78.53 

_  Specific  gravity, 

1.620 

1.664 

I-655 

1.642 

1.665 

'Chestnut,  over  ^  inch  round  mesh, 

8.44 

0.98 







Pea    coal,  between    y$    inch    and  9-16  \ 
inch  round  mesh,                                 J 

60.65 

6.85 

0.31 

1.50 

1.  21 

No.   i   Buckwheat,  between  9-16   inch  \ 
and  y%  inch  round  mesh,                    j 

21.70 

57-72 

4-76 

4-58 

2.60 

Sizing 
test.  < 

No.   2    Buckwheat,    between    Ji    inch  \ 
and  3-16  inch  round  mesh,                 J 

3-68 

28.74 

66.57 

17-75 

3'-94 

No.  3   Buckwheat,  between   3-16   inch  \ 
and  3-32  inch  round  mesh,                 J 

1.40 

2-39 

19.87 

45-95 

49-57 

Between  3-32  inch  and  1-16  inch,  some-  ) 
times  allowed  in  No.  3  Buckwheat,  \ 

4-13 

1.49 

2-39 

19.79 

6.31 

^.Dust  through  1-16  inch  round  mesh, 



1.83 

6.10 

10.43 

8-37 

Slate    \ 

Pure  coal,  specific  gravity  below  1.70, 

92.00 

76.18 

78.28 

86.98 

83.85 

test,     / 

Slate  and  bone,  specific  gravity  above  1.70, 

8.00 

23.82 

21.72 

13.02 

16.15 

1  Furnace   for    Burning    Small    Anthracite    Coals. 
Engineers,  Vol.  XXII.     1894. 


Eckley    B.    Coxe.      Am.    Inst.    Mining 


7 2  MECHANICAL   DRAFT. 

Tests  i  to  4  inclusive  were  made  upon  two  Stirling  water-tube  boilers,  each 
having  1,725  square  feet  of  heating  surface  and  55  square  feet  of  grate;  while 
No.  5  was  made  upon  a  cylinder  boiler  with  drums  and  connecting  tubes,  the 
total  heating  surface  being  1,862  square  feet  and  68.75  square  feet  of  grate. 
All  the  boilers  were  equipped  with  Coxe  travelling  grates  and  forced  draft  under 
the  grate. 

Other  tests,1  upon  a  return  tubular  boiler  equipped  with  Coxe  stoker,  in- 
dicated the  relative  efficiencies  of  various  fuels  as  presented  in  Table  No.  47. 

Table  No.  47. —  Relative  Efficiencies  of  Small  Anthracite  Coals. 


Pounds  of  Water  per    |     Pounds  of  Water  per 
Pound  of  Coal  from  and)   Pound  of  Combustible 
at  212°.  from  and  at  212°. 


Buckwheat,  8.77 

Rice  (No.  2  Buckwheat),  9.05 


Culm  (Pea,  Buckwheat,  Rice,  Barley,  Dust), 
Barley  (No.  3  Buckwheat), 


8.74 
8-39 


11.07 
11.18 
11.19 
10.89 


Influence  of  Air  Supply.  —  Although  the  loss  or  waste  that  occurs  through  an 
improper  amount  of  air  supplied  for  combustion  is  not  properly  chargeable  to 
the  fuel,  yet,  under  practical  conditions,  this  factor  exerts  an  important  influence 
upon  their  relative  efficiency.  As,  with  all  other  conditions  the  same,  the 
amount  of  air  passing  through  the  fire  is  with  chimney  draft  liable  to  change 
according  to  the  atmospheric  conditions  and  the  methods  of  firing  and  operat- 
ing the  dampers,  especial  care  is  necessary  to  secure  uniformity  and  equality  of 
air  volume  in  all  tests  which  are  to  be  compared.  Such  uniformity  can  be 
readily  maintained  in  plants  operated  by  mechanical  draft. 

In  reality  the  intensity  of  the  draft  whereby  the  requisite  volume  of  air  is 
supplied  is  of  the  greatest  importance,  as  is  clearly  shown  in  succeeding  chapters. 
This  makes  possible  thick  fires  and  the  most  economical  distribution  of  the 
fuel.  "A  considerable  saving  of  fuel,"  as  asserted  by  Hutton,2  "may  be 
effected  by  the  employment  of  well-arranged  forced  draft,  .  .  .  and  greater 
power  may  be  obtained  with  less  size  and  number  of  boilers  than  with  boilers 
having  combustion  with  natural  draft." 


1  Some  Thoughts  on  the  Economical  Production  of  Steam,  with  Special  Reference  to  the 
Use  of  Cheap  Fuel,  by  a  Miner  of  Coal.     Eckley  B.  Coxe.     Trans.  New  England  Cotton  Manu- 
facturers' Association.     April,  1895. 

2  Steam-Boiler  Construction.     Walter  S.  Hutton.     London,  1891. 


MECHANICAL   DRAFT. 


73 


The  method  of  calculating  the  theoretical  amount  of  air  required  for  the 
complete  combustion  of  any  fuel  has  already  been  explained.  If  less  than  this 
amount  is  supplied,  a  certain  portion  of  the  carbon  passes  off  unconsumed  and 
forms  smoke ;  while  a  part  of  the  remainder,  being  insufficiently  supplied  with 
oxygen,  forms  carbonic  oxide,  the  product  of  incomplete  combustion.  If  the 
air  be  supplied  in  excess  of  that  necessary  for  perfect  combustion,  there  is  a 
definite  loss,  which  is  twofold  in  its  character :  First,  the  excess  of  air  entering 
the  furnace  is  heated  by  the  burning  fuel,  thereby  lowering  the  temperature  of 
the  mixture  of  gases  and  air  below  that  which  would  prevail  if  the  gases  only 
were  present.  As  a  consequence,  the  rate  of  absorption  of  heat  by  the  water  is 
reduced,  for  it  is  dependent  upon  the  difference  in  temperature  between  the 
water  and  the  gases.  Second,  owing  to  larger  volume  and  higher  velocity  the 
temperature  of  the  mixture  of  gases  and  air  escaping  io  the  chimney  is  higher 
than  would  be  the  case  if  there  were  no  excess  of  air  ;  while  the  increased  volume 
is  such  that  the  total  amount  of  heat  thus  carried  away,  without  exerting  any 
useful  effect,  is  greatly  increased.  In  other  words,  paradoxical  as  it  may  seem, 
the  larger  the  volume  of  air  supplied,  the  higher  will  be  the  temperature  of  the 
escaping  gases. 

Influence  of  the  Frequency  of  Firing.  —  While  the  rate  at  which  any  given 
coal  should  be  fed  to  the  furnace  is  largely  dependent  upon  the  character  of 
the  coal,  nevertheless  it  is  doubtless  true  that  in  most  cases  it  is  fed  in  too 
large  quantities  and  at  too  long  intervals.  The  natural  result  is  a  series  of 
decided  and  almost  critical  changes  in  the  condition  of  the  fire,  to  be  compared 
to  the  effect  of  eating  a  large  amount  of  food  once  a  day  instead  of  a  smaller 
amount  at  a  greater  number  of  times.  It  is  evident  that  efficiency,  as  regards 
the  frequency  of  firing,  is  entirely  dependent  upon  the  fireman,  and  hence  for 
favorable  conditions  the  advantages  of  a  mechanical  stoker. 

For  the  purpose  of  ascertaining,  so  far  as  possible,  the  relative  results  of 
different  rates  of  firing,  M.  Burnat '  conducted  a  series  of  experiments  extend- 
ing over  eight  weeks,  with  the  same  fireman  and  the  same  boiler. 

The  general  results  are  presented  in  Table  No.  48.  The  advantage  of  the 
smallest  charge  of  13  pounds  over  the  maximum  of  55  pounds  is  in  the  first 
series  3.03  per  cent,  and  in  the  second  8.19  per  cent.  These  results  are  obtained, 
notwithstanding  the  fact  that  with  the  more  frequent  firing  the  doors  were  more 
frequently  opened.  A  boiler  arranged  so  that  the  damper  became  closed  or 
nearly  so  when  the  door  was  opened  showed  upon  test  an  increased  evapora- 
tion of  14  to  15  per  cent  due  to  this  arrangement. 


Bulletin  de  la  Societe  Industrielle  de  Mulhouse,  Vol.  XLVI.     1876. 


74 


MECHANICAL   DRAFT. 


Table  No.  48.  —  Influence  of  Frequency  of  the  Charges  of  Coal. 


?1 

Temperature 
of  Feed  Water. 

Temperature 
of  Hot  Gases. 

Pounds  of  Coal 
Consumed. 

1 

Jhi 

KIND  OF  COAL. 

Iti 

to 

Ml 

^ 

£      • 

M 

*H 

"o  "  M'O 

^=LU 
§J 

W     K 

H 

1| 

!1| 

§ 
E 

||| 

U 

1 

111 

r* 

01 

| 

Oi 

r 

202 

900 

213° 

8490 

4210 

225 

10.8 

13-3 

12.9 

9.87 

Ronchamp,  Nut,       ^ 

202 

87 
87.5 

224 

227 

840 
844 

426 
414 

225 
225 

10.8 
10.8 

26.6 

39-2 

13-4 

12.8 

9-59 
9-59 

I 

202 

86 

226 

835 

421 

225 

10.8 

55-4 

12.8 

9-58 

Ronchamp, 

226 

870 

87 

226 

7.790 
784- 

3960 
410 

225 
225 

10.8 
10.8 

55-o 
41.1 

16.1 
14.6 

8.91 
9.18 

large  and  small,      1 

2OI 

87-5 

229 

795 

410 

225 

10.8 

28.0 

14.7 

'I 

202 

86 

228 

853 

489 

225 

10.8 

15.0 

12.6 

9.64 

Loss  on  Account  of  Smoke.  —  The  loss  resulting  from  the  formation  of  smoke 
is  absolute  ;  for  it  is  equivalent  to  directly  robbing  the  fire  of  a  part  of  the  fuel 
from  which  not  only  has  no  heating  effect  been  secured,  but  upon  which  heat 
has  actually  been  wasted  in  raising  it  to  the  temperature  of  the  escaping  flue 
gases.  Notwithstanding  the  prevailing  impression  as  to  the  great  losses  due  to 
the  formation  of  smoke,  the  actual  waste  is  comparatively  insignificant,  as  is 
shown  by  the  following  results  of  carefully  conducted  experiments  by  Mr.  J.  C. 
Hoadley.1 

During  an  entire  week  gas  was  drawn  from  the  flue  of  the  boiler  under  test, 
passed  through  a  gas  meter  and  thence  through  a  muslin  strainer  at  the  bottom 
of  a  vessel  of  water.  When  a  sufficient  quantity  of  the  gases  had  been  passed 
the  water  was  evaporated,  and  the  residuum  was  dried  and  weighed.  The  coal 
used  was  bituminous,  of  the  following  average  composition  :  — 


Carbon     . 

Hydrogen 

Ash 

Water       . 

Oxygen 

Nitrogen 

Sulphur    . 


81.03  per  cent. 

3-84  " 

7.19  « 

0.63  " 

4-49  " 

2.00  " 

0.82  " 


Warm-Blast  Steam-Boiler  Furnace.     J.  C.  Hoadley.     New  York,  1886. 


MECHANICAL   DRAFT. 


75 


The  total  quantity  of  coal  burned  during  the  week  was  12,890  pounds,  the  total 
quantity  of  flue  gases  reduced  to  72°  being  4,263,119  cubic  feet,  and  the  total 
amount  of  solid  matter  42.63  pounds,  as  shown  by  the  test.  There  was,  there- 
fore, present  in  solid  form  in  the  flue  gases  only  — 

=  -°°33  =  °'33  per  cent 

of  the  matter  originally  present  in  the  coal.  As  the  gray  color  of  the  substance 
thus  recovered  indicated  that  it  was  not  more  than  half  carbon,  it  is  evident 
that  under  the  conditions  of  the  test  the  proportion  of  carbon  which  was 
actually  carried  off  in  black  smoke  was  about  one-sixth  of  one  per  cent  of  the 
original  coal. 

MM.  Scheurer-Kestner  and  Meunier  '  passed  flue  gases  through  asbestos, 
upon  which  the  particles  of  carbon  were  deposited,  and  found  that,  with  good 
fire  and  draft  and  an  air  supply  of  257  cubic  feet  per  pound  of  coal,  one  half 
of  one  per  cent  of  the  carbon  of  the  coal  was  lost  as  smoke  particles.  In  a 
second  trial,  with  poor  fire  and  draft  and  only  118  cubic  feet  of  air  per  pound 
of  coal,  the  deposited  carbon  was  one  per  cent  of  the  total  contained  in  the 
coal.  This  latter  is  to  be  taken  as  a  maximum,  the  conditions  being  decidedly 
adverse,  and  is  more  than  would  be  produced  in  ordinary  practice.  The  infer- 
ence from  these  experiments  must  be  that  the  average  loss  of  carbon  in  the 
solid  form  as  smoke  may  be  taken  not  to  exceed  one  half  to  three  quarters  of 
one  per  cent. 

Loss  on  Account  of  Carbonic  Oxide.  —  The  loss  of  efficiency  which  ensues 
from  the  escape  of  carbonic  oxide  unconverted  into  carbonic  acid  is  due  to  the 
much  smaller  amount  of  heat  given  out  upon  the  incomplete  combustion  of 
carbon  into  carbonic  oxide.  While  carbon  burned  to  carbonic  acid  generates 
14,650  B.  T.  U.,  the  same  quantity  burned  to  carbonic  oxide  gives  out  only 
4,400  B.  T.  U.  For  every  pound  of  carbon  which  passes  off  in  the  form  of  car- 
bonic oxide,  there  is,  therefore,  a  loss  of  14,650  —  4,400  =10,150  B.  T.  U.,  or  — 

— '        =  0.6028  =  60.28  per  cent. 
14,650 

The  ultimate  effect  of  the  formation  of  carbonic  oxide  upon  the  total  heat  of 
combustion  can  be  best  illustrated  by  reference  to  the  calculation  under  "  Heat 
of  Combustion,"  in  Chapter  III.  If,  instead  of  the  entire  0.80  pound  of  carbon 
having  been  perfectly  burned,  only  0.70  pound  had  entered  into  combustion 
with  oxygen  to  form  carbonic  acid,  and  the  remaining  o.io  pound  had  entered 


Bulletin  de  la  Societe  Industrielle  de  Mulhouse.     1868,  1869. 


76  MECHANICAL   DRAFT. 

into  combustion  as  carbonic  oxide,  the  calculation  would  have  been  as  follows : 

Heat,  in  B.  T.  U.,  =  14,650  x  0.70  -j-4>4°°  x  o.io  -{-62,100  (0.05  —     '  —^ ) 

=  13,590  B.T.U.; 

showing  that  there  would  be  a  loss  in  the  calorific  value  of  the  coal  of  about 
7  per  cent.  Had  only  half  of  the  carbon  been  completely  burned,  the  loss 
would  have  been  about  18  per  cent.  But  such  a  loss  even  as  that  first  in- 
stanced does  not  usually  occur  continuously  in  any  well  designed  and  operated 
boiler  furnace.  In  fact,  the  loss  from  this  source  appears  to  be  largely  over- 
estimated in  most  cases. 

Three  days'  continuous  analysis  of  the  flue  gases  from  the  boiler  plant  of  the 
B.  F.  Sturtevant  Company  at  Jamaica  Plain,  Mass.,  operating  under  induced 
draft,  failed  to  indicate  the  presence  of  an  amount  of  carbonic  oxide  greater 
than  one  tenth  of  one  per  cent,  or  sufficiently  large  to  be  identified  by  the 
refined  apparatus  employed  for  the  purpose.  In  Hoadley's  warm-blast  furnace 
tests  the  carbonic  oxide  "never  in  the  day  time  exceeded  half  of  one  per  cent, 
and  rarely  exceeded  half  that  small  quantity  when  the  dampers  were  open,  for 
six  weeks  together." 

From  the  analyses  of  the  chimney  gases  of  124  boiler  tests  made  at  the  Indus- 
trial Exhibition  at  Diisseldorf  in  iSSo,1  the  average  amount  of  carbonic  oxide 
by  volume  was  found  to  be  0.747  per  cent.  In  all  cases  but  one  bituminous 
coal  was  used.  The  air  supply  was  almost  constant ;  the  minimum  amount  of 
nitrogen  shown  in  any  of  the  tests  being  79.3  per  cent,  and  the  maximum  84.08 
per  cent,  but  ranging  in  most  cases  between  80  and  81  per  cent.  Omitting  four 
analyses  in  which  the  carbonic  oxide  ranged  from  3  to  5.3  per  cent,  the  average 
presence  of  this  gas  in  the  remaining  120  tests  appears  to  have  been  0.637  Per 
cent. 

There  are  some  conditions  of  boiler  practice,  however,  in  which  such  good 
results  do  not  obtain,  and  in  which  more  or  less  serious  losses  may  occur,  largely 
due  to  an  insufficiency  of  air.  It  should  be  clearly  understood,  however,  that 
the  same  amount  of  air  is  not  always  required.  When  the  coal  upon  the  grate 
is  thoroughly  ignited  the  minimum  supply  is  necessary,  but  when  the  fire  is 
suddenly  thickened  and  cooled  by  additional  coal  there  is  a  demand  for  addi- 
tional supply  for  the  purposes  of  combustion,  together  with  a  tendency  to  clog 
the  passages  through  which  the  air  has  previously  passed,  and  thereby  to  pre- 
vent complete  combustion  at  the  surface  of  the  fire.  At  this  time,  for  perfect 


i  Die  Untersuchungen  an  Dampfmaschinen  und  Dampfkesseln,  und  an  einigen  Rheinischen 
und  Westfalischen  Kohlensorten  auf  der  Gewerbe-Ausstellung  in  Diisseldorf  in  1880,  heraus- 
gegeben  von  H.  v.  Reiche  und  F.  Booking,  Aachen,  1881.  Velag  von  J.  A.  Meyer. 


MECHANICAL   DRAFT. 


77 


Table  No'.  49.  — Carbonic  Oxide  Produced  by  Excessive  Firing. 


TIME. 

Pounds  of 
Coal 
Thrown  on 
Grate. 

Carbonic 
Acid 
in  Chimney 
Gases. 

Carbonic 
Oxide 
in  Chimney 
Gases. 

Ratio  of 
Carbon  in 
Carbonic  Ox- 
ide to  Total 
Carbon. 

Pounds 
of  Air  per 
Pound  of 
Coal. 

Pounds  of 
Coal  Burned 
Each 
Half-Hour. 

Ratio  of  Loss 
by  Carbonic 
Oxide  to  Full 
Power  of  Coal. 

Per  cent 

H.      M. 

Pounds. 

CO2. 

Per  cent  CO. 

Per  cent. 

Pounds. 

Pounds. 

Per  cent. 

6:15  a.  m. 

200 

6=45 

200 

7:15 

2OO 

7=45 

200 

8:15 

20O 

3:45 

200 

9:00 

5-12 

2.54 

43.80 

33-2 

83.81 

'   27.84 

9=15 

200 

9oO 

5-55           2.99 

4S-85            29-5 

93-75 

29.14 

9=45 

200 

1  0:00 

7-79 

3-99 

44.63 

21.4 

129.24 

28.37 

10:15 

2OO 

10:30 

7.70 

4.61 

48.47 

20.1 

137.60 

30.81 

10:45 

2OO 

11:00 

7.82 

4.70 

48-57 

I9.8 

139.68 

30.88 

11:15 

200 

11:30 

8.01 

4.81 

48.55 

19-3 

143-3° 

30.86 

12:00  m. 

19-3 

I43-30 

12:30  p.  m. 

15.21 

0.25 

2.52 

'9-3 

143-3° 

1.  60 

12:45 

200 

1:00 

2O.O5 

137.94 

1:30 

14.11 

0.21 

2.28 

20.8 

132.96 

1.49 

2:00 

21.05 

137-94 

2:30 

13.62 

o-33 

3-67 

21.3 

129.85 

2.31 

2:45 

20O 

3:00 

14.50 

0.48 

4-95 

19.4 

142.56 

3-14 

3:3o 

13.18 

0.29 

3-34 

22.0 

125.34                       2.12 

3=45 

200 

4:00 

14.96 

0.38 

3-84 

19-3 

I43.30 

2-44 

4=30 

14.18 

0.41 

4-35 

20.3 

136.25 

2.76 

4:45 

200 

5:00 

13.01 

0.41 

4.72 

22.0 

125.34 

3-oo 

Mean  quantity  of  air    . 

. 

21.653 

Mean  of  all  but  first  two       .         . 

20.36 

Mean  ratio  of  loss  ;  first  six,  per  cent 

29.65 

Mean  ratio  of  loss  ;  last  eight,  per  cent 

2-36 

7 8  MECHANICAL   DRAFT. 

conditions,  more  air  should  be  admitted,  as  is  possible  under  the  positive  action 
of  mechanical  draft  arranged  to  operate  automatically.  But  such  additional 
supply  is  only  necessary  for  a  comparatively  short  time. 

The  effect  of  excessive  firing  is  practically  equivalent  to  the  reduction  of  draft 
and  air  supply  for  the  regular  amount  of  coal,  and  is  conducive  to  the  produc- 
tion of  carbonic  oxide.  This  was  well  exemplified  by  Mr.  J.  C.  Hoadley  in  a 
special  test,1  the  results  of  which  are  presented  in  Table  No.  49.  As  is  evident 
from  the  items  in  the  first  and  second  columns,  the  firing  was  at  first  very  rapid, 
decreasing  as  the  day  passed.  During  the  morning  200  pounds  were  fired  every 
half-hour,  while  during  the  afternoon  the  same  amount  was  fired  hourly.  Gas 
samples  were  not  taken  until  9  A.  M.,  after  which  time  the  excess  of  carbonic 
oxide  is  noticeable  until  the  slower  firing  began,  when  the  carbonic  oxide  is  im- 
mediately reduced,  while  the  carbonic  acid  is  practically  doubled.  The  ratio 
of  loss  by  carbonic  oxide,  indicated  in  the  last  column,  is  of  special  interest. 

Admission  of  Air  above  the  Fire.  —  Since  the  days  of  C.  Wye  Williams  and 
his  famous  work  on  the  "  Combustion  of  Coal  and  the  Prevention  of  Smoke," 
the  introduction  of  air  above  or  beyond  the  fire  has  been  one  of  the  favorite 
methods  adopted  in  the  attempt  to  perfect  the  combustion  of  coal  and  prevent 
the  appearance  of  smoke.  In  so  far  as  the  admission  of  air  for  the  purpose  of 
perfecting  the  combustion  is  concerned,  the  results  in  the  way  of  water  evapora- 
tion per  pound  of  fuel  should  naturally  be  looked  to  as  the  true  indication  of 
its  efficiency.  That,  under  certain  conditions,  increased  efficiency  can  be  thus 
obtained  is  evidenced  in  many  tests.  As  a  rule  the  gain  is  not  large,  but  is 
greatest  with  bituminous  coals,  whose  large  percentage  of  volatile  constituents 
tends  more  readily  to  the  formation  of  both  carbonic  oxide  and  smoke. 

Table  No.  50,  compiled  from  carefully  conducted  tests  by  Mr.  George  H. 
Barrus,2  serves  to  show  both  the  percentage  of  gain  and  loss  by  such  contriv- 
ances, the  character  of  which  is  indicated  under  the  designating  letters,  as 
follows  :  — 

A.  Air  conducted  direct   from    outside    the  setting  to    the    interior    of   the 
bridge  wall  and  discharged  therefrom  through  perforations  in  its  top  covering. 
The  air  thus  supplied  mingles  with  the  lower  strata  of  burning  gas  as  it  skims 
over  the  bridge. 

B.  Air  supplied  first  to  a  pipe  laid  in  the  bridge  wall  and  thence  to"  per- 
forated cast-iron  globes  resting  upon  its  top.     A  more  thorough  mixture  of  air 
and  gases  is  thus  secured.     A  jet  of  steam  is  employed  to  increase  the  volume 


Warm-Blast  Steam-Boiler  Furnace.     New  York,  1886. 
Boiler  Tests.     George  H.  Barrus.     Boston,  1891. 


MECHANICAL    DRAFT. 


79 


of  air  which  would  be  drawn  in  by  natural  means,  and  the  steam  thus  supplied 
mingles  with  the  air. 

C.  Air  supplied  through  perforations  in  the  top  of  the  bridge  wall  and  in  the 
sides  of  the  furnace  after  first  passing  through  a  series  of   passages  running 
lengthwise  of  the  walls  of  the  setting. 

D.  Air  admitted  through  perforated  tiles  in  the  sides  of  the  furnace  of  a 
vertical  tubular  boiler,  after  having  passed  up  and  down  through  ducts  in  the 
walls. 

E.  Air  admitted  in  similar  manner  in  the  case  of  a  horizontal  tubular  boiler, 
after  having  passed  through  side-wall  heating  flues,  as  in  the  case  of  D. 

Table  No.  50.  — Effect  of  Admitting  Air  above  the  Fire. 


GA 

IK. 

Lc 

ss. 

KIND  OF  COAL. 

•o 

C^j 

1  i 

Id 

•s  i 

£j 

loj 

a 

M 

C 

r° 

£  1 

r° 

n 

A 

Cumberland, 

5-9 

6.2 

A 

Anthracite,  broken, 

0.0 

I.O 

A 

Two  parts  Pea  and  Dust  and  one  part  Cumberland, 

2.0 

4-7 

B 

Cumberland, 

8.4 

8.0 

B 

Anthracite,  broken, 

1.9 

3-7 

C 

Two  parts  Pea  and  Dust  and  one  part  Culm, 

2.0 

o.o 

D 

(  Two  parts  Anthracite  Screenings  and  one  part  ) 
(           Cumberland,                                                      ) 

4-3 

2-3 

E 

Three  parts  Pea  and  Dust  and  one  part  Cumberland 

4-5 

4-7 

F 

Nova  Scotia, 

I.O 

'•5 

F.  Air  supplied  through  perforations  in  the  top  of  the  bridge  wall  and  inside 
of  furnace. 

Mr.  Barrus  concludes  from  these  tests  that  "  a  considerable  advantage 
attends  the  admission  of  air  above  the  fuel  when  bituminous  coal  is  employed, 
the  amount  of  gain  depending  somewhat  upon  the  method  employed.  There 
is  no  advantage  in  the  system  where  mixtures  of  anthracite  screenings  and 
bituminous  coal  are  used,  if  carried  out  according  to  either  the  first  or  fourth 
methods  [A  and  C,  as  here  designated],  and  finally,  little  or  no  benefit  is 
derived  when  anthracite  coal  is  burned." 

It  will  be  observed  that  four  of  the  boilers  here  designated  were  provided 
•with  certain  devices  for  heating  the  air  before  its  admission  to  the  fire.  The 


So  MECHANICAL    DRAFT. 

gain  in  efficiency,  by  such  an  arrangement,  is  at  the  best  but  slight,  and  when 
the  attempt  is  made  to  heat  by  such  means  all  of  the  air  supplied  both  below 
and  above  the  fire,  this  plan  is  found  to  be  totally  inadequate.  Such  increase 
in  temperature  of  the  air  supply  as  may  be  secured  by  proper  and  adequate 
appliances  properly  concerns  the  efficiency  of  the  boiler,  under  which  heading 
it  will  be  discussed.  Although  it  is  undoubtedly  true  that  heating  the  air 
intensifies  the  chemical  affinities  between  the  air  and  the  fuel,  it  is  doubtful  if 
under  ordinary  conditions  the  effect  is  sufficient  to  be  noticeable  in  boiler  prac- 
tice. This  is,  however,  independent  of  the  economy  resulting  from  the  abstrac- 
tion of  waste  heat  from  the  flue  gases. 

The  best  results  from  the  admission  of  air  above  the  fire  appear  to  be 
secured  when  it  is  discharged  in  fine  jets  into  the  furnace  chamber.  For  the 
accomplishment  of  such  results  other  draft  than  that  of  the  chimney  is  neces- 
sary. It  is  thus  that  mechanical  draft  becomes  an  important  factor,  both  in 
thus  furnishing  the  required  air  supply  and  in  overcoming  the  added  resistance 
which  results  from  any  attempt  to  preheat  the  air. 

Loss  on  Account  of  Excess  of  Air.  —  As  has  already  been  stated,  it  is  a  prac- 
tical impossibility  so  to  distribute  the  amount  of  air  theoretically  required  for 
the  perfect  combustion  of  a  given  fuel  as  to  secure  the  ideal  result.  In  prac- 
tice, notwithstanding  the  fact  that  all  excess  means  loss  of  efficiency  of  the 
fuel,  it  is  necessary  to  supply  enough  additional  air  to  ensure  complete  combus- 
tion even  if  a  loss  is  occasioned  thereby.  What  this  excess  should  be,  from  an 
economical  standpoint,  can  only  be  determined  by  practical  experiment ;  and  it 
will  be  found  to  vary  with  the  character  of  the  fuel,  its  rate  of  combustion,  the 
temperature  of  the  supply  and  the  intensity  of  the  draft. 

For  the  purpose  of  illustration  an  anthracite  coal  may  be  taken,  containing 
about  80  per  cent  of  carbon,  with  an  amount  of  inherent  oxygen  and  hydrogen 
so  small  in  quantity  and  of  so  little  effect  that,  under  the  circumstances,  it  may 
be  neglected.  The  amount  of  air  required  for  the  complete  combustion  of  one 
pound  of  carbon  having  already  been  shown  to  be  11.3  pounds,  that  necessary 
for  the  combustion  of  one  pound  of  this  coal  is,  therefore,  11.3  x  0.8  =9.04 
pounds,  as  is  also  evident  by  the  following  calculation :  — 

Carbon,  0.8  Oxygen,  2.133 

Oxygen,  2-I33  Nitrogen,          6.906 


Carbonic  acid,          2.933  Air,  9-039 

Nitrogen,  6.906  Carbon,  0.80 


Products,  9-839  Products,         9-839 


MECHANICAL   DRAFT. 

The  specific  heat  of  the  products  may  be  found,  as  follows,  to  be  0.2358. 

Weight.          Specific  Heat.  B.  T.  U. 

Carbonic  acid       .         .         .          2.933    x    0.2169   =     -6362 
Nitrogen       ....         6.906    x    0.2438  =    1.6837 


81 


9-839 


2.3199 


This  value  of  the  specific  heat  may  be  taken  without  appreciable  error  to 
apply  to  the  products  of  combustion  of  carbon,  no  matter  what  the  excess  of 
air  supplied.  It  is,  therefore,  evident  that  i.o  X  0.2358  =0.2358  pounds  of 
water  may  be  heated  through  i  degree  by  the  cooling  of  the  gases  through 
i  degree. 

By  the  methods  here  and  previously  described,  Table  No.  51  has  been  calcu- 
lated for  the  purpose  of  showing  clearly  the  effect  upon  the  weight  and  ideal 
temperature  of  the  gaseous  products  of  combustion  and  their  relative  volume, 
resulting  from  the  supply  of  air  in  various  amounts  in  excess  of  that  theoreti- 
cally necessary  for  combustion. 

Table  No.  51.  —  Effects  of  Excess  of  Air. 


Excess  of  Air,  in  per 
chemically  required. 

Total  Weight  of  Air. 
Pounds. 

Total  Weight  of 
Products  of  Perfect 
Combustion. 
Pounds. 

Ideal  Temperature 
above  62°  in 
Heart  of  Fire. 
Degrees. 

Relative  Volume  of 
Gases  at  Ideal  Temper- 
ature above  62°. 

0 

9.04 

9.84 

5.053 

1.  00 

5° 

T3'56 

14.36 

3»46l 

1.04 

75 

15.82 

16.62 

2,491 

1.  06 

IOO 

1  8.08 

18.88 

2,633 

1.09 

125 

20.34 

21.14 

2.351 

I.  II 

150 

22.60 

23.40 

2,124 

1-13 

!75 

24.86 

25.66 

L937 

MS 

200 

27.12 

27.92 

1,780 

1.17 

Evidently,  as  the  total  volume  of  gases  is  heated  in  each  case  by  the  prod- 
ucts of  only  one  pound  of  coal,  the  total  heat  is  constant ;  but  because  of  the 
greater  volume  absorbing  that  heat  the  ideal  temperature  decreases  as  the  air 
supply  increases. 

For  simplicity,  the  effect  of  the  greater  specific  gravity  of  carbonic  acid  has 
been  neglected  in  the  calculation,  and  the  products  of  combustion  have  been 
taken  as  of  constant  density,  at  constant  temperature,  with  varying  amounts  of 
air  supplied. 


£2  MECHANICAL   DRAFT. 

The  first  effect  of  increasing  the  air  supply  is  to  lower  the  temperature  of 
the  products  of  combustion,  as  has  already  been  shown  and  as  is  further  indi- 
cated in  Table  No.  51.  Were  it  not  for  the  intervening  boiler  these  gases 
would,  therefore,  pass  to  the  chimney  in  increased  volume  but  with  decreased 
temperature.  The  absorption  of  heat  by  a  boiler  of  the  ordinary  proportions 
serves,  however,  to  bring  about  a  result  that  is  apparently  paradoxical.  In 
ordinary  practice  a  boiler  of  good  design  is  capable,  under  the  usual  conditions, 
of  utilizing  so  much  of  the  heat  that  the  gases  pass  to  the  chimney  at  a  tem- 
perature somewhere  between  450°  and  650°.  An  increase  in  the  air  supply  to 
the  furnace  in  connection  with  such  a  boiler  actually  results  in  raising  the  tem- 
perature of  the  escaping  gases,  as  may  be  thus  explained. 

It  is  generally  accepted  that  in  the  case  of  a  steam  boiler  the  amount  of 
heat  transmitted  from  the  gases  to  the  water,  per  degree  difference  between  the 
gases  and  the  boiler  plates  and  tubes,  is  practically  unifgrm  for  various  differ- 
ences of  temperature.  In  other  words,  it  is  practically  proportional  to  the 
difference  in  temperature.  It  is,  further,  evidently  true  that  in  the  case  of 
moving  gases  the  amount  of  heat  transmitted  will  be  proportional  to  the  time 
they  remain  in  contact  with  the  given  surface.  Owing  to  the  lower  temperature 
and  greater  density,  their  volume  in  the  furnace  is,  therefore,  not  greatly  in- 
creased by  a  moderate  increase  in  the  air  supply,  as  is  shown  in  Table  No.  51. 

But,  as  they  pass  across  the  heating  surface  and  are  cooled,  their  volume, 
and  hence  their  velocity,  decreases,  and  as  their  'temperature  approaches  that 
of  the  admitted  air  so  does  their  volume.  In  other  words,  the  average  rate  of 
flow  becomes  more  nearly  proportional  to  the  original  air  volume.  As  a  conse- 
quence, the  cooler  gases  resulting  from  the  admission  of  a  larger  volume  of  air 
have  less  time  in  which  to  give  up  their  heat,  and  are,  therefore,  cooled  less  in 
proportion  to  their  temperature  than  are  the  hotter  gases  resulting  from  the 
admission  of  a  smaller  air  volume.  In  addition,  these  cooler  gases,  because  of 
the  less  difference  between  their  temperature  and  that  of  the  boiler,  give  up 
their  heat  less  rapidly ;  the  final  result  of  these  two  influences,  by  which  the 
transmission  of  heat  is  retarded,  being  that  the  gases  accompanying  the  larger 
admission  of  air  leave  the  boiler  at  a  higher  temperature  than  those  which 
result  from  the  admission  of  a  smaller  amount.  Evidently,  under  these  condi- 
tions the  evaporative  power  of  the  boiler  must  be  decreased. 

M.  Burnat  conducted  a  series  of  experiments,  showing  that  the  temperature 
of  the  escaping  gases  increased  with  the  supply  of  air.  The  boiler,  provided 
with  special  heaters,  had  a  heating  surface  of  475  square  feet,  with  a  grate  of 
18  square  feet;  the  quantity  of  coal  consumed  was  293  pounds  per  hour,  or 
1 6  pounds  per  square  foot  of  grate.  These  results  are  given  in  Table  No.  52. 


MECHANICAL   DRAFT.  83 

Table  No.  52.  —  Effect  of  Increased  Air  Supply  upon  Temperature  of  Escaping  Gases. 


DAY. 

Cubic  Feet  of  Air  at  62°  per 
Pound  of  Coal. 

Average  Temperature  of  the  Gases 
Leaving  the  Boiler. 

First, 

272 

624° 

Second, 

198 

601 

Third, 

168 

550 

Fourth, 

124 

487 

The  theoretical  loss  of  efficiency,  when  air  is  supplied  in  excess  and  the  prod- 
ucts of  combustion  escape  at  different  temperatures  above  the  atmosphere,  is 
exemplified  in  Table  No.  53,  the  coal  having  a  heat  value  of  11,720  B.  T.  U. 
This  relates  to  the  combustion  of  one  pound  of  coal.  An  air  supply  100  per 
cent  in  excess  of  that  chemically  required,  and  a  temperature  of  escaping  gases 
450°  above  the  atmosphere,  represents  fairly  well  the  ordinary  conditions  of 
boiler  practice  with  chimney  draft,  under  which  it  will  be  noted  that  the  loss  is 
no  less  than  17.0  per  cent.  A  chimney  requires  a  high  and  wasteful  temperature 
of  gases  to  produce  the  draft,  but  this  is  unnecessary  with  mechanical  means  for 
draft  production.  A  moderate  reduction  to  75  per  cent  excess  air  supply  and 
300°  would  show  an  economic  gain  of  7  per  cent. 

Table  No.  53.  —  Loss  of  Efficiency  Due  to  Excess  of  Air,  and  Temperature  of 
Escaping  Gases  above  Atmosphere. 


11 


Temperature  of  Escaping  Gases  above  Atmosphere. 


« 

300° 

350° 

4oo° 

45°°' 

500° 

550° 

Excess  of  Air  ir 
of  that  chemical! 

tJ 

51 

«j 

Is 

Loss  in  per  cent 
of  Total  Heat 
Value  of  Coal. 

Total  B.  T.  U. 
in  Gases. 

Loss  in  per  cent 
of  Total  Heat 
Value  of  Coal. 

Total  B.  T.  U. 
in  Gases. 

IB 

*3o 

$ 

'k 
l> 

Loss  in  per  cent 
of  Total  Heat 
Value  of  Coal. 

Total  B.  T.  U. 
in  Gases. 

Loss  in  per  cent 
of  Total  Heat 
Value  of  Coal. 

Total  B.  T.  U. 
in  Gases. 

Loss  in  per  cent 
of  Total  Heat 
Value  of  Coal. 

0 

695 

5-9 

812 

6.9 

928 

7-9 

1,044 

8.9 

1,160 

9-9 

1,276 

10.9 

5° 

1,016 

8.7 

1.185 

IO.I 

*,354 

u-5 

1,524 

I3.0 

1,693 

14.4 

1,862 

15-9 

75 

1,176 

1  0.0 

1,372 

II-7 

1,568 

'3-3 

1,764 

I5.0 

',959 

I6.7 

2>I55 

18.4 

100 

!»336 

11.4 

1.558 

J3-3 

1,781 

15.2 

2,003 

I/.O 

2,226 

19.0 

2,448 

20.9 

125 

1.495 

12.7 

1,745 

14.9 

1.994 

17.0 

2,243 

I9.I 

2,492 

21.2 

2,742 

23-4 

150 

1,655 

14.1 

I>931 

i6.S 

2,207 

18.8 

2,483 

21.  1 

2,759 

23-5 

3,035 

25-9 

175 

1,815 

iS-S 

2,118 

1  8.0 

2,420 

2O.6 

2,715 

23.1 

3,o25 

25.8 

3,328 

28.4 

200 

r>975 

1  6.8 

2,304 

19.6 

2,633 

22.4 

2,957 

25.2 

3,291 

28.0 

3,621 

30-8 

MECHANICAL   DRAFT. 


As  bearing  upon  the  evaporation  of  the  boilers,  the  results  embodied  in  Table 
No.  54  emphatically  indicate  the  loss  due  to  an  increase  in  the  air  supply. 
These  tests,  by  M.  Burnat,1  were  upon  boilers  of  various  types.  The  air  chem- 
ically required  for  perfect  combustion  was  130  cubic  feet  per  pound  in  the  case 
of  the  Ronchamp  coal. 

From  these  results  it  would  appear  that  the  best  practice  consisted  in  keep- 
Table  No.  54.  — Effect  of  Air  Supply  on  the  Efficiency  of  Fuel. 


^ 

Water 

IB 

Air  at  62°  sup- 

evaporated from 

BOILER. 

COAL. 

JS'5 

Coal  per  Hour. 

plied  per  Poun< 

and  at  212° 
per  Pound  of 

°*W 

Coal. 

Per  cent 

Pounds. 

Cubic  feet. 

Pounds. 

Heating  surface  =513 
square  feet. 

Grate  surface  =  14.2 
square  feet. 

Ronchamp 
mixed. 

100 
IOO 
107 

112 

"3 

33° 

33° 
33° 
330 
33° 

219 

216 

174 
I48 
127 

7.09 
7-08 
7.62 

8.00 
8.06 

• 

IOO 

284 

222 

5-46 

104 

285 

229 

5.67 

Heating  surface  —  301 
square  feet. 

Sarrebriick 
slack,  very 
inferior. 

1  08 

112 
112 

276 

257 
242 

200 

J45 
T53 

5-93 
6.1  1 
6.13 

108 

237 

207 

5.92 

no 

234 

126 

6.01 

Heating  surface  =  475 
square  feet. 

Sarrebriick 

IOO 

1  08 

367 
370 

290 
264 

5.26 
5-67 

Grate  surface  =  18 

slack,  very 
inferior. 

112 

114 

III 

375 
361 

367 

190 
141 
196 

5.88 
6.03 
5.86 

square  feet. 

IOI 

316 

121 

5-32 

Half 

Heating  surface  =  291 
square  feet. 

Ronchamp 
slack  and 
Sarrebriick 

IOO 

1  08 

107 

280 
263 
259 

190 
I69 

1S2 

6.60 
7.10 
7.09 

slack. 

110 

260 

123 

7.26 

Bulletin  de  la  Societe  Industrielle  de  Mulhouse,  Vol.  XXX.     1859-60. 


MECHANICAL   DRAFT. 


ing  the  volume  of  air  admitted  as  small  as  possible.  But  the  result  of  such 
supply,  although  increasing  the  efficiency,  even  though  imperfectly  burning  the 
coal,  would  cause  a  dull  fire  with  abundant  smoke  and  meet  with  serious  prac- 
tical objection  on  account  of  the  difficulty  of  maintaining  it  under  varying  con- 
ditions of  demand.  It  would,  in  fact,  be  a  fire  requiring  the  utmost  care  and 
attention  on  the  part  of  the  fireman,  and  liable  to  fail  to  raise  steam  when 
suddenly  required. 

Summary  of  Influences  Affecting  the  Efficiency  of  Fuel.  —  The  relative  im- 
portance of  the  principal  influences  which  have  just  been  discussed  and  the 
manner  in  which  they  are  exerted  is  very  clearly  shown  by  an  analysis  of  the 
results  of  combustion  of  a  given  quantity  of  coal.  For  illustration,  there  have 
been  taken  100  pounds  of  anthracite  coal  of  the  composition  shown  in  the 
accompanying  tabular  view.  It  is  assumed  that  double  the  theoretical  amount 
of  air  is  supplied,  that  the  atmosphere  contains  a  normal  amount  of  moisture, 
that  two  pounds  of  carbon  remain  unconsumed  and  pass  to  the  ashpit,  that 
neither  carbonic  oxide  nor  smoke  is  formed,  that  the  coal  and  air  have  an  origi- 
nal temperature  of  60°  when  they  enter  the  furnace,  that  the  chimney  gases  have 
a  temperature  of  500°,  and  that  the  ash  has  a  temperature  of  450°. 

The  total  heat  of  100  pounds  of  the  fuel  is  shown  to  be  1,313,080  B.  T.  U., 
but,  owing  to  direct  loss  to  the  ashpit,  the  heat  generated  amounts  to  only 
1,283,780  B.  T.  U.  The  Tabular  View  indicates  the  relative  weights  of  the  vari- 
ous products  of  combustion,  and  the  accompanying  Table  No.  55  shows  the 
heat  losses  from  these  sources,  together  with  their  method  of  calculation.  It  is 

Table  No.  55.  — Heat  Losses  Incident  to  the  Combustion  of  100  Pounds 
Anthracite  Coal. 


HEAT  LOSSES. 

Number 
of  B.  T.  U. 

Per  cent  of 
Total  Heat 
of  Fuel. 

By  water  =  [(212  —  60)  X  wt.]  +  965.7  X  wt.+  [sp.  heat  X  (500—  2i2)Xwt], 

37,012-5 

2.83 

By  carbonic  acid  =  wt.  X  sp.  heat  X  (500  —  60), 

27,994.2 

2.13 

By  nitrogen  =  wt.  X  sp.  heat  X  (500  —  60), 

158,452.8 

I2.O7 

By  free  oxygen  =  wt.  X  sp.  heat  X  (500  —  60), 

21,973.6 

1.67 

By  ash  =  wt.  X  sp.  heat  X  (450  —  60), 

1,105.7 

0.08 

By  carbon  in  ash  =  wt.  X  sp.  heat  X  (450  —  60)  +  wt.  X  14,650, 

29,488.3 

2.24 

By  carbonic  oxide  =  wt.  X  sp.  heat  X  (500  —  60)  +  wt.  X  4,400, 





Total  heat  lost  exclusive  of  loss  by  radiation, 

276,027.! 

21.02 

Theoretically  possible  evaporation  in  pounds  of  water  from  and  at 

per  pound  of  combustible  utilized,  f 

Theoretically  possible  evaporation  in  pounds  of  water  from  and  at  2120  | 
per  pound  of  fuel  utilized,  J 


12-73 


86 


MECHANICAL   DRAFT, 


Tabular  View  Showing  Results  of  Combustion  of  100  Pounds  of  Anthracite  Coal  with 
Twice  the  Theoretical  Amount  of  Air  Required. 


POUNDS. 

'  Water,       . 

2.00  , 

Ash, 

11.50  

i 

i 

, 

100  Ibs. 

Carbon,     . 

82.00  j 

i 

—  !-  -, 

of  coal. 

Hydrogen, 

2.00  

-H    ' 

Oxygen,     . 

1.60-  - 

Jl 

Nitrogen,  . 

0.90  

:  i 

-••;  WASTE  PRODUCTS  IN  CHIMNEY. 

1     |     ; 

Entering 
furnace. 

1     I                                    p        .             Per  cent 
,     |                                    Pounds'       by  Weight. 
\-  -  -  —  Steam,           29.50             1.46 

i     1    i 

f  Oxygen  for  CO2, 

213-33  — 

-M-j—  CQ»          293.33         14.48 

Oxygen  for  H2O, 

14.40-  - 

-H         ;--Nitrogen,  1475-27           72.82 

1         : 

1929.83  Ibs. 

Oxygen  for  CO, 

00.00  — 

—  ^—  —  ^       .                oo.oo           oo.oo 

1            '• 

of  air. 

Ox          f 

yg    •     . 

-27-73  — 

—  ;  —  Oxygen,       227.73            11.24 
i         • 

Nitrogen,  . 

474-37  

i 

^  Water,       . 

9.50  -  - 

WASTE  PRODUCTS  IN  ASHPIT. 

Pounds.       by  wefght. 

(  Ash,           11-50           85.18 

(  Carbon,       2.00           14.81 

(  Weight  of  C  X  14,650 

=  82X14,650=   .         .         .         1,201,300  B.  T.  U. 

Total  heat  of  fuel.  |  ^  of  R  _  (WtofO)  x  62>ioo  =  s  _  ^  x  ^  =  1  1  ^        u 

1,313,080           « 

f  80  X  14,650  = 

.       ...        .        .         1,172,000  B.  T.  U. 

Heat  generated.        2_  1.6   x 

MECHANICAL    DRAFT. 


evident  that  under  the  assumed  conditions  21.02  per  cent  is  irretrievably  lost, 
and  that,  neglecting  the  loss  by  radiation  from  brickwork,  the  efficiency  of  the 
fuel  is  78.98  per  cent.  With  the  given  fuel  these  losses  can  only  be  lessened 
by  decreasing  the  air  supply,  preventing  the  loss  of  some  of  the  carbon  to  the 
ashpit,  and  by  proper  means  lowering  the  final  temperature  of  the  escaping  gases. 
The  total  loss  through  moisture  in  coal  and  air,  ash  in  coal,  and  carbon  in  ash 
amounts  to  5.15  per  cent. 

Commercial  Efficiency  of  Coals.  —  The  cost  of  producing  a  given  amount  of 
steam  is  the  ultimate  criterion  by  which  the  efficiency  of  any  fuel  must  be 
judged.  The  commercial  efficiency  not  only  concerns  the  amount  of  water 
evaporated  by  a  pound  of  fuel,  but  is  directly  dependent  upon  the  following 
items  of  expense:  first;  interest,  rent,  taxes  and  insurance  on  the  cost,  and 
the  depreciation  of  the  plant  ,•  second,  repairs ,-  third,  cost  of  water  from  which 
the  steam  is  generated  ,-  fourth,  labor  ,•  fifth,  getting  rid  of  the  ashes ;  sixth, 
cost  of  the  fuel  in  the  boiler  house. 

The  cost  of  water  may  be  considered  practically  constant  in  the  comparison 
of  fuels  in  a  given  boiler  plant,  but  the  items  of  interest,  depreciation  and 
repairs  are  to  a  great  extent  directly  dependent  upon  the  fuel ;  for  with  the 
cheaper  fuels  more  expensive  contrivances  are  usually  necessary,  and  under  the 
same  conditions  the  output  of  the  plant  decreases  with  the  quality  of  the  fuel. 
This  is  quite  clearly  shown  in  Table  No.  46,  in  which  the  water  evaporated  per 
square  foot  of  heating  surface  grows  less  as  the  quality  of  the  fuel  degenerates. 
It  is  still  further  shown  by  Table  No.  56,  in  which  the  horse-power  of  various 

Table  No.  56.  —  Effect  of  Quality  of  Fuel  upon  Output  of  Boiler. 


RATED  HORSE-POWER  OF  BOILER. 

54 

74                    87 

129                  140 

270 

Cumberland, 

6O.O 

143-8 

1054 

94.0 

214.6 

Anthracite,  broken, 

53-9 

I°5-S 

84.0 

192.3 

103.8 

196.1 

Pea, 

149.2 

One  part  Screenings  and  one  part  Cum-  ) 
berland,                                                j 

95.1 

Three  parts    Screenings  and   one  part  ) 

204.8 

Cumberland,                                          j 

Two  parts  Pea  and  Dust  and  one  part  ) 
Cumberland,                                          ( 

38.5 

82.2 

118.1 

Forty-four   parts    Pea   and    Dust   and  ) 
thirty-seven  parts  Culm,                      ( 

I57-I 

88  MECHANICAL   DRAFT. 

boilers,  tested  by  Mr.  George  H.  Barrus1  under  somewhat  different  draft  condi- 
tions, is  compared  with  their  rated  power  when  using  various  kinds  of  coal. 
This  decreased  power  is  directly  due  to  the  reduced  combustion  per  square  foot 
of  grate,  which  may  be  counteracted  to  a  certain  extent  by  increasing  the  grate 
surface. 

The  largest  and  most  important  factor,  however,  is  the  cost  of  the  fuel  itself, 
which  should  be  measured  not  by  the  number  of  pounds  but  by  the  available 
heat  units  obtained  for  a  given  price.  In  this  cost  are  properly  included  the 
transportation  charges,  the  expense  of  getting  the  coal  into  the  boiler  house 
and  putting  it  into  the  furnace  as  well  as  taking  out  and  carrying  away  the  ash. 
Practically  all  of  these  costs  are  directly  dependent  upon  the  weight  of  the 
coal,  regardless  of  the  number  of  heat  units  it  is  capable  of  developing.  But 
the  net  cost  depends  not  upon  the  number  of  units  in  the  coal  but  upon  the 
number  that  can  be  utilized  under  the  given  conditions.  The  haulage  of  ashes 
becomes  so  important  in  some  cases  that  it  is  found  more  economical  to  pay 
a  higher  price  for  a  coal  containing  less  ash  rather  than  go  to  the  necessary 
expense  of  teaming  the  ashes  a  considerable  distance. 

As  the  cost  of  transportation  of  the  coal  is  practically  dependent  upon  the 
weight  and  independent  of  the  character  of  the  coal,  the  proportional  difference 
in  price  which  may  rule  at  the  mine  may  be  almost  extinguished  when  an  equal 
charge  is  added  for  the  transportation  of  each.  Thus  No.  2  buckwheat  may 
prove  a  very  economical  fuel  when  utilized  at  the  mine,  where  it  may  be  pur- 
chased at  25  cents  per  ton,  although  its  direct  calorific  efficiency  is  low  and  it 
requires  a  special  form  of  furnace  for  satisfactory  combustion.  No.  i  buck- 
wheat coal  costing  50  to  60  cents  under  the  same  conditions  may,  however, 
prove  to  be  a  close  rival  because  of  its  higher  efficiency  and  the  greater  ease 
with  which  it  may  be  consumed.  But  this  difference  of  100  to  120  per  cent  in 
cost  becomes  reduced  to  a  difference  of  only  1 1  to  15  per  cent  when  a  trans- 
portation charge  of  $2.00  per  ton  is  added  to  each.  Both  of  these  coals, 
although  requiring  a  larger  boiler  plant  for  the  same  aggregate  evaporative 
results,  may,  after  transportation  charges  are  added,  prove  on  the  whole  more 
economical  than  larger  sized  anthracite  of  higher  efficiency  but  at  higher  prices. 

A  comparison  of  the  losses  shown  in  Table  No.  55,  reduced  to  the  commer- 
cial relation  when  different  kinds  of  coal  are  used,  serves  to  make  the  preceding 
clear.  This  comparison  is  presented  in  Table  No.  57,  showing  the  relation 
between  an  anthracite  buckwheat  costing  50  cents  per  ton  at  the  mines,  the 
same  coal  when  transportation  charges  of  $2.00  have  been  added,  and  pea  coal 


Boiler  Tests.     George  H.  Barrus.     Boston,  1891. 


MECHANICAL   DRAFT. 


89 


costing  $1.50  at  the  mines,  with  the  same  expense  for  transportation  as  the 
buckwheat.  In  each  case  the  coal  is  assumed  to  be  burned  with  double  the 
theoretical  amount  of  air,  while  the  loss  by  radiation,  which  might  amount  to 
from  4  to  20  per  cent,  has  not  been  considered  This  loss,  if  known,  should  be 
deducted  from  the  amount  here  given  as  actually  utilized.  The  cost  is  based 
in  the  four  cases  upon  the  theoretical  evaporation  of  10.44  pounds  of  water 
from  and  at  2 12°  per  pound  of  coal,  as  given  in  Table  No.  55.  In  practice 
these  results  would  undoubtedly  have  to  be  corrected,  relatively  as  well  as 
directly,  because  of  the  higher  evaporation  probable  with  the  pea  coal. 

Table  No.  57.  —  Commercial  Value  of  the  Losses  Incident  to  Burning  100  Pounds 

of  Coal. 


LOSSES. 

KIND  AND  PRICE  OF  COAL. 

Buckwheat  Coal.                      Pea  Coal. 

$0.50 

52.50                 $1.50 

$35° 

By  water, 
By  carbonic  acid, 
By  nitrogen, 
By  free  oxygen, 
By  ash, 
By  carbon  in  ash, 

$0.00063 
0.00048 
0.00269 
0.00037 
O.OOOO2 
O.OOO50 

$0.00315 
0.00238 
0.01347 
O.OOI86 
0.0009 
0.0025 

$O.OOl89 
0.00143 
0.00808 
O.OOII2 
0.00005 
O.OOI5O 

$0.00442 
0.00333 
0.01886 
O.OO26I 
O.OOOI2 
O.OO35O 

Total  loss,  not  including  radiation  from  brickwork, 

0.00469 

0.02245 

0.01407 

0.03284 

Actually  utilized,  including  radiation  from  brickwork, 
Cost  of  fuel  to  evaporate  100  pounds  of  water  from  ) 
and  at  2120,                                                             \ 

0.01763 
0.00214 

O.o88l6 
O.OIO09 

0.05289 
0.0064  ! 

O.I234I 
0.01497 

Based  upon  the  cost  of  the  coal  and  firing,  the  following  will  serve  to  illus- 
trate the  economy  that  may  be  secured  by  burning  a  cheaper  fuel  of  lower 
efficiency.  In  a  certain  plant  equipped  with  horizontal  tubular  boilers  and 
down-draft  furnaces,  an  evaporation  of  8.5  pounds  of  water  from  and  at  212° 
was  regularly  obtained.  The  coal  used  was  Illinois  slack,  costing  $1.40,  deliv- 
ered under  the  boilers.  This  makes  the  cost  about  8.3  cents  per  1,000  pounds 
evaporated  under  these  conditions.  Had  the  best  picked  anthracite  coal,  cost- 
ing $4.80  per  ten,  under  existing  market  rates,  been  used,  and  had  it  evaporated 
the  generous  amount  of  12  pounds  of  water  from  and  at  212°,  the  cost  would 
have  been  20  cents  per  1,000  pounds. 


9° 


MECHANICAL   DRAFT. 


Taking  the  relative  values  given  in  Table  No.  43  and  the  prices  ruling  at 
the  time  and  in  the  locality  of  the  tests,  and  following  Mr.  Barrus'  method  of 
comparison,  the  cost  of  coal  of  the  various  kinds  necessary  to  generate  1,000 
horse-power  may  be  calculated.  A  horse-power,  as  applied  as  a  standard  of 
boiler  capacity,  is  elsewhere  explained  as  being  equivalent  to  the  evaporation  of 
34.488  pounds  of  water  from  and  at  212°.  The  daily  production  of  steam  dur- 
ing a  ten  hours'  run  of  a  1,000  horse-power  plant  would,  therefore,  be  1,000 
X  10  x  34.488=344,880  pounds  from  and  at  212°.  The  results  of  calcula- 
tion thus  obtained  are  embodied  in  Table  No.  58,  as  well  as  those  showing  the 
cost  of  labor  and  the  total  cost  of  coal  and  labor.  In  estimating  the  labor  the 
assumption  is  made  that  in  the  case  of  anthracite  coal  of  broken,  chestnut  and 
pea  sizes  the  labor  is  performed  by  two  day  firemen,  one  night  fireman  and  two 

Table  No.  58.  —  Cost  of  Coal  and  Labor  for  a  Day's  Run  of  Ten  Hours,  1,000  Horse- 
power Plant. 


1 

£  . 

'  S~ 

<§!-•§ 

111 

£ 

5  . 

1 

11 

£  ~ 

—  I 

PH 

Hi 

II 

£S 
g'3 

§ 
0 

C32 

<q 

^ 

,*"=  °;j 

111 

B 
< 

£° 

£ 

Weight  of  coal  used  in  ten  hours  (2,240  Ibs.  \ 
—  i  ton),  tons,                                            \ 

13-9 

15-7 

16.4 

16.4 

17.1 

17.4 

18.3 

Cost  of  coal  per  ton  of  2,240  Ibs.,                S 

4.56 

5-65 

6.13 

3-72 

3-29 

3-74 

3.28 

Cost  of  coal  used  in  ten  hours,                       $ 

63-38 

88.70 

100,53 

61.05 

56.26 

62.25 

60.02 

Cost  of  labor  per  day,                                       $ 

9.50 

7-75 

7-75 

10.75 

10.75 

7-75 

9.50 

Total  cost  of  coal  and  labor  per  day,            $ 

72.88 

96-45 

108.28 

71.80 

67.01 

73.00 

69.52 

Relative  practical  heat  value  (Cumberland  | 

IOO 

89 

85 

'85 

82 

80 

76 

=  100),  per  cent,                                      ( 

Relative  cost  of  coal  per  ton  (Cumberland  j 
=  100),  per  cent,                                      j 

100 

124 

134 

82 

72 

81 

72 

Relative  total  cost  of  coal  and  labor  per  | 
day  (Cumberland  =  100),  per  cent,        ( 

IOO 

132 

149 

99 

92 

IOO 

95 

helpers,  and  in  the  case  of  the  bituminous  coal  one  additional  fireman  is 
.required ;  while  for  the  mixed  coals  one  fireman  and  one  helper  additional  are 
necessary.  It  is  further  assumed  that  the  wages  of  the  firemen  are  $1.75  per 
day  and  of  the  helpers  $1.25  per  day.  From  this  table  it  is  evident  that  under 
the  prevailing  prices  there  is  practically  no  choice  between  the  Cumberland, 
anthracite,  pea,  and  mixture  of  pea  and  dust  with  Cumberland ;  while  the  Nova 
Scotia  culm  and  the  other  mixture  do  not  fall  far  behind  in  efficiency.  The 
anthracite  broken  and  chestnut  coals  are,  however,  not  only  most  expensive 
per  ton,  but  are  also  the  least  efficient  from  a  commercial  standpoint.  The 


MECHANICAL    DRAFT. 


relative  values  in  per  cent  —  based  on  Cumberland  as  100 — serve  to  show 
clearly  the  stated  relations  existing  between  the  different  coals.  Based  upon 
the  cost  of  the  coal  alone,  the  rate  of  evaporation  must  vary  inversely  as  the 
cost  in  order  to  secure  equivalent  results  in  water  evaporated  per  unit  of  cost. 
This  principle  has  been  carried  out  in  the  calculations  of  Table  No.  59.  Thus, 
if  coal  at  $3.50  per  ton  has  an  evaporative  power  of  unity,  it  will.be  necessary 
for  one  pound  of  coal  at  $5.00  per  ton  to  evaporate  1.43  times  as  much  water 
to  produce  the  same  commercial  result;  or  if  coal  costing  $1.50  per  ton  is  sub- 
stituted for  coal  costing  $4.00  per  ton,  the  cost  per  pound  of  water  evaporated 
will  be  the  same  if  the  latter  fuel  evaporates  0.38  as  much  as  the  former. 

Table  No.  59.  — Rates  of  Evaporation  for  Equivalent  Cost  of  Coal. 


COST  OF  COAL  PER  TON. 

COST  OF  COAL 

PER  TON. 

$0.50 

£1.00 

$I;50 

$2.00 

$2.50 

$3-0° 

$3-50 

$4.00 

$4-5° 

$5.00 

$5.50 

$6.00 

$6.50 

$7.oo 

$0.50 

I.OO 

2.0O 

3.00 

4.00 

5.00 

6.00 

7.00 

8.00 

9.OO 

IO.OO 

I  I.OO 

I2.OO 

13.00 

14.00 

1.  00 

0.50 

I.OO 

1.50 

2.OO 

2.50 

3.00 

3-5° 

4.00 

4-5° 

5.00 

5-50 

6.00 

6.50 

7.00 

1.50 

o-33 

0.67 

I.OO 

!-33 

1.67 

2.00 

2-33 

2.67 

3-00 

3-33 

3-67 

4.00 

4-33 

4.67 

2.00 

0.25 

0.50 

0.75 

I.OO 

1-25 

1.50 

i-75 

2.00 

2.25 

2.50 

2-75 

3.00 

3-25 

3-5° 

2.50 

O.20 

0.40 

0.60 

0.80 

I.OO 

1.20 

1.40 

1.  60 

1.  80 

2.00 

2.20 

2.40 

2.60 

2.80 

3-00 

0.17 

o-33 

0.50 

0.67 

0.84 

I.OO 

1.17 

i-33 

I.S0 

1.67 

1.83 

2.00 

2.17 

2-33 

3-5° 

0.14 

0.29 

0.43 

0.57 

0.71 

0.86 

I.OO 

1.14 

1.28 

i-43 

i-57 

I.7I 

1.86 

2.OO 

4-00 

0.13 

0.25 

0.38 

0.50 

0.63 

o-75 

0.88 

I.OO 

1-13 

1-25 

1.38 

I.50 

1.63 

r-75 

4-5° 

O.I  I 

0.22 

°-33 

0.45 

0.56 

0.67 

0.78 

0.89 

I.OO 

i.  ii 

1.22 

x-33 

1.44 

r-55 

5.00 

0.10 

O.20 

0.30 

0.40 

0.50 

0.60 

0.70 

0.80 

0.90 

I.OO 

I.IO 

i.  20 

1.30 

1.40 

5-5° 

0.09 

0.18 

0.28 

0.36 

0-45 

o-SS 

0.64 

0.73 

0.82 

0.91 

I.OO 

1.09 

1.18 

1.27 

6.00 

0.08 

0.17 

0.25 

o-33 

0.42 

0.50 

0.58 

0.67 

0-75 

0.83 

0.92 

I.OO 

i.  08 

1.17 

6.50 

0.07 

0.15 

0.23 

0.31 

0-39 

0.46 

0-54 

0.62 

0.69 

0.77 

0.85 

0.92 

I.OO 

i.  08 

7.00 

0.07 

0.14 

O.2I 

0.29 

0.36 

0.43 

0.50 

0.57 

0.64 

0.71 

0.79 

0.86 

0-93 

I.OO 

Influence  of  Mechanical  Draft.  —  Intense  draft  is  one  of  the  most  important 
factors  in  the  utilization  of  cheap  fuels ;  hence  the  value  of  mechanical  draft. 
Its  influence  will  be  discussed  later  at  length,  but  it  is  here  proper  to  present  at 
least  one  illustration  of  the  increased  economy  in  fuel  resulting  from  its  use. 

At  the  United  States  Cotton  Company's  mills,  at  Central  Falls,  R.  I.,  is  a 
boiler  plant  consisting  of  3  Babcock  &  Wilcox  boilers  of  335  horse-power 
each, —  a  total  of  1,005  horse-power.  The  draft  is  furnished  by  a  Sturtevant 
blower,  which  forces  the  air  to  the  ashpits,  and  whose  speed  is  automatically 
regulated  by  a  special  device,  so  that  the  volume  of  air  and  intensity  of  draft 


92  MECHANICAL    DRAFT. 

are  continually  changing  to  suit  the  varied  conditions  and  requirements  of  the 
fire.  Before  this  mechanical  draft  plant  was  put  into  operation,  the  fuel 
employed  was  George's  Creek  Cumberland  coal,  costing  $4.00  per  ton  of  2,200 
pounds,  delivered  at  the  boiler  room.  Since  the  fan  with  automatic  control  has 
been  in  use,  the  quality  and  price  of  the  fuel  has  been  reduced  to  a  mixture 
of  about  70  to  75  per  cent  of  No.  2  buckwheat,  20  to  25  per  cent  of  yard 
screenings,  and  5  to  10  per  cent  of  Cumberland,  costing  $2.62  per  ton.  This 
change  has  reduced  the  fuel  cost  per  indicated  horse-power  on  the  engine  by 
$0.001235  per  horse-power  per  hour.  At  the  horse-power  developed  -by  the 
engines,  the  present  saving  per  week  is  over  $126.00,  when  all  fuel,  including 
that  for  banking  fires,  is  taken  into  account.  Comment  is  unnecessary. 

Prevention  of  Smoke.  —  The  tendency  of  a  coal  to  produce  smoke  increases 
with  the  volatile  combustible  matter  which  enters  into  its  composition.  Pure 
carbon  and  coke  are  smokeless,  and  the  best  anthracite  coal  is  practically  so, 
but  the  bituminous  coals  are  as  a  rule  distinguished  for  their  smoke-producing 
qualities.  As  has  already  been  shown,  the  actual  amount  of  unconsumed  car- 
bon passing  away  from  a  well-constructed  boiler  in  the  form  of  smoke  seldom 
if  ever  exceeds  one  per  cent  of  the  total  amount  of  carbon  in  the  coal.  Neve^r- 
theless,  because  of  the  visible  effect  of  even  such  a  small  amount,  the  "  smoke 
nuisance  "  is  widespread  and  of  serious  consequence  in  some  localities.  Many 
have  been  the  devices  presented  and  applied  for  overcoming  this  evil,  although 
but  few  have  met  with  ultimate  success. 

As  smoke  is  a  result  of  incomplete  combustion,  its  prevention  must  be  sought 
through  the  provision  of  an  ample  supply  of  air,  with  sufficient  intensity  of 
draft  and  the  maintenance  of  a  high  temperature  of  the  fuel  bed.  Unless  the 
proposed  preventative  device  meets  these  requirements  it  has  little  hope  of 
success. 

The  contrivances  which  have  been  applied  for  the  purpose  may  be  broadly 
classed  as  follows  :  — 

I.  Mechanical  or  forced  draft. 

II.  Arrangements  for  admission  of  air  above  the  fire,  under  which  may  be 
included  steam  jets  for  inducing  a  flow  of  air. 

III.  Firebrick  arches  or  checker  work,  placed  over  the  bridge  wall  or  near 
the  end  of  the  fireplace,  for  the  purpose  of  mixing  and  heating  the  gases. 

IV.  Hollow  walls  for  preheating  air. 

V.  Coking  arches  or  chambers  constructed  in  front  of  the  fireplace,  whence 
the  coke  is  pushed  to  the  rear  as  the  volatile  matter  is  distilled  off. 

VI.  Double  combustion,  whereby  part  or  all  of  the  gases  are  passed  a  second 
time  through  the  fuel. 


MECHANICAL   DRAFT.  93 

VII.  Down-draft  furnaces  in  which  air  is  admitted  above  the  grate  and  the 
gases  pass  down  through  it  and  thence  to  the  heating  surface. 

VIII.  Automatic  stokers. 

With  coals  of  moderate  smoking  qualities  mechanical  draft  in  its  simplest 
application,  by  its  power  to  furnish  an  adequate  amount  of  air  under  a  pressure 
sufficient  to  cause  it  to  pass  readily  through  the  fuel,  meets  all  the  requirements 
of  a  smoke  preventative,  even  when  all  of  the  air  is  admitted  beneath  the  grate. 
With  excessively  smoky  coals,  however,  a  portion  of  the  air  may  be  omitted 
above  the  fuel.  The  best  results  are  obtained  when  this  air  enters  the  furnace 
under  the  influence  of  a  positive  means  like  mechanical  draft,  in  a  series  of  jets, 
by  which  it  is  forced  to  commingle  with  the  gases  as  they  rise  from  the  bed  of 
fuel.  Owing  to  the  tendency  of  cold  air  thus  admitted  to  chill  the  fire  and 
actually  increase  the  amount  of  smoke,  it  is  desirable,  when  rapid  combustion 
takes  place,  that  the  air  be  preheated  to  a  considerable  degree. 

The  steam  jet  has  also  been  employed  to  induce  a  flow  of  air  which,  mixed 
with  steam  and  thus  heated,  is  forced  either  beneath  or  above  the  fire,  as  the 
case  may  be.  While  the  admixture  of  steam  incident  to  this  method  of  draft 
production  lessens  the  tendency  to  clinker,  nevertheless  its  cost  of  operation 
is  much  greater  than  that  of  the  fan  blower,  as  will  be  shown  later.  Evidently 
all  of  the  steam  thus  admitted  must  be  raised  to  the  temperature  of  the  escap- 
ing gases,  thereby  reducing  the  efficiency  of  the  fuel. 

Mr.  J.  C.  Hoadley1  made  clear  the  inefficiency  of  preheating  devices  where 
the  draft  is  not  sufficiently  strong  to  cause  the  necessary  movement  of  air,  and 
the  inadequacy  of  passages  in  brick  settings  as  means  of  heating  the  air  prior  to 
its  admission  above  the  fire.  In  the  boiler  under  test  these  passages  were 
formed  in  the  brick  setting  of  the  back  and  sides,  in  the  rear  of  the  bridge  wall, 
and  communicated  with  openings  at  the  bridge  wall  and  in  the  side  walls  of  the 
furnace.  Sliding  dampers  were  provided  to  regulate  the  admission  of  air ;  but, 
as  stated  by  Mr.  Hoadley,  "careful  and  repeated  experiments  and  observations 
proved  that  these  dampers  could  never  be  opened  without  checking  the  draft 
through  the  fuel  and  lowering  the  temperature  of  the  fire ;  and  it  is  not  impos- 
sible that  a  very  slight  leakage  through  the  closed  dampers  may  have  lowered 
the  efficiency  of  the  boilers." 

As  regards  the  arrangement  "  for  heating  air  or  '  superheating  it '  (whatever 
superheating  may  be  supposed  to  mean  when  applied  to  a  permanent  gas), 
.  .  .  no  good  was  ever  found  to  result  from  this  system  of  flues ;  indeed  it  is 
doubtful  if  any  considerable  quantity  of  air  ever  passed  through  the  flues  at  all, 


•Warm-Blast  Steam-Boiler  Furnace.     J.  C.  Hoadley.     New  York,  1886. 


94  MECHANICAL   DRAFT. 

although  some  must  have  flowed  in  when  the  dampers  were  opened,  since  the 
resistance  of  the  open  flue,  circuitous  as  it  was,  could  hardly  have  been  so  great 
as  that  of  the  coal  on  the  fire  grates." 

With  a  temperature  of  about  2,000°  immediately  above  the  fire  in  the  ordi- 
nary furnace,  it  is  evident  that  any  device  which  heats  the  air  but  a  few  degrees 
above  the  temperature  of  the  atmosphere  can  have  no  practical  effect  upon  the 
combustion,  and  that  some  more  elaborate  arrangement  is  necessary.  As  such 
devices  pertain  more  properly  to  the  efficiency  of  the  boiler,  they  will  be  dis- 
cussed under  that  head. 

Firebrick  arches  and  coking  chambers  are  both  serviceable  in  preventing 
the  formation  of  smoke,  but  require  very  careful  management. 

As  the  inflammable  gas  (CO)  and  the  uncons-umed  carbon  together  seldom 
exceed  two  per  cent  of  the  total. gases,  any  attempt  to  burn  them  again,  as 
by  "double  combustion,"  is  futile.  Apparent  success  is  due  to  the  admission 
of  additional  air. 

The  down-draft  furnace,  owing  to  reversal  of  the  usual  direction  of  move- 
ment of  the  gases,  requires  a  water  grate  to  withstand  the  intense  heat.  The 
practical  features  of  such  construction  generally  make  it  impossible  to  prevent 
considerable  of  the  fuel  dropping  through  the  grate.  To  avoid  loss  from  this 
source  an  auxiliary  grate  is  usually  provided,  upon  which  this  fuel  may  be  con- 
sumed. Even  with  a  forced  fire  and  careless  firing  such  an  arrangement  appears 
to  insure  a  good  smoke  record.  The  conditions  are  such,  however,  that  greater 
draft  is  required  than  with  the  ordinary  type  of  furnace,  so  that  mechanical 
draft  is  often  found  to  be  of  especial  importance. 

As  a  rule,  automatic  stokers  are  introduced  for  reasons  other  than  the  preven- 
tion of  smoke,  although,  by  their  uniformity  of  feeding  and  their  frequent  appli- 
cation of  the  coking  principle  in  their  construction,  they  are  capable,  under 
favorable  conditions,  of  giving  good  results.  They  do  not,  however,  readily 
handle  caking  or  clinkering  coals,  and  usually  require  coal  of  the  large  sizes, 
and  hence  are  restricted  in  their  application. 

With  ordinary  coal  and  hand-firing  the  prevention  of  smoke  is  largely 
dependent  upon  the  fireman,  irrespective  of  any  special  appliances ;  for  these, 
no  matter  how  excellent  their  character,  are  in  course  of  time  likely  to  be 
neglected.  Many  devices  applied  for  this  purpose,  or  for  increasing  the  effi- 
ciency, have  shown  favorable  results  merely  because  they  compelled  greater 
attention  on  the  part  of  the  fireman  in  the  care  of  boilers  that  were  previously 
worked  with  marked  inefficiency.  Beyond  certain  factors,  such  as  a  sufficiency 
of  draft,  over  which  he  can  have  no  control,  a  good  fireman  is,  after  all,  the  most 
important  factor  in  increasing  efficiency  and  preventing  smoke. 


CHAPTER   VI. 
EFFICIENCY   OF   STEAM   BOILERS. 

Measure  of  Efficiency.  —  The  practical  efficiency  of  a  boiler  and  that  of  the 
fuel  consumed  in  connection  with  it  are  interdependent.  That  is,  the  attainable 
efficiency  of  the  fuel  is  dependent  upon  the  design  and  operation  of  the  boiler ; 
while  the  efficiency  of  a  given  boiler  is  a  direct  measure  of  the  amount  of  heat 
derived  from  the  fuel  employed.  It  has  long  been  the  custom  to  compare 
boilers  upon  the  basis  of  the  number  of  pounds  of  water  evaporated  by  each  per 
pound  of  fuel  or  combustible  burned.  Of  course,  efficiencies  measured  thus 
vary  greatly  in  different  types  of  boilers ;  but,  broadly  stated,  it  is  undoubtedly 
true  that  the  rate  of  evaporation  per  pound  of  coal  from  feed-water  at  60°  into 
steam  of  80  pounds  gauge  pressure  is  in  general  below  8  pounds.  This  is 
equivalent  to  9.56  pounds  from  and  at  212°.  Indeed,  8  pounds  of  dry  steam 
under  the  above  conditions  is  a  fair  result,  8.25  pounds  a  good  result,  8.5  pounds 
very  good  and  9  pounds  about  the  best  usually  attainable.  This  latter  amount 
corresponds  to  10.74  pounds  from  and  at  212°,  is  equivalent  to  69  per  cent  of 
the  full  calorific  power  of  carbon,  and  is  for  coal  of  five-sixths  carbon  a  high 
result. 

Results  thus  compared  are,  however,  liable  to  be  deceptive, —  in  some  cases 
intentionally  so, —  because  of  the  wide  variation  in  the  quality  of  coal.  Thus, 
by  means  of  picked  coal  of  high  calorific  power,  an  exceptionally  high  evapora- 
tion may  be  obtained  and  used  to  advocate  the  merits  of  a  given  boiler.  Less 
attention  is  generally  given  to  the  quality  of  the  coal  than  to  the  amount  of 
water  evaporated  per  pound,  and  the  fact  is  not  always  recognized  that  a  poor 
boiler  tested  with  good  coal  may  actually  give  a  greater  evaporation  than  a  good 
boiler  with  poor  coal.  The  possibility  of  obtaining  such  results  is  rendered 
evident  by  Table  No.  60,  showing  three  sets  of  assumed  but  perfectly  practical 
conditions. 

If  the  measure  of  efficiency  were  to  be  based  solely  upon  the  evaporation  per 
pound  of  coal,  boiler  B  would  be  selected.  But  it  is  equalled  in  efficiency  by 
boiler  C,  if  the  efficiency  be  measured  by  the  proportion  of  available  heat 
utilized,  although  it  evaporates  1.33  pounds  less  water  because  of  the  poorer 
quality  of  the  coal.  Whether  it  would  be  commercially  the  more  efficient  of  the 


9 6  MECHANICAL   DRAFT. 

two  would  depend  upon  the  relative  cost  of  the  two  kinds  of  coal,  and  that 
having  a  heat  value  of  9,500  B.  T.  U.  would  have  to  cost  about  14  per  cent  less. 
Based  upon  water  evaporated,  boiler  A  appears  more  efficient  than  boiler  C,  but 
measured  by  the  amount  of  heat  absorbed  the  latter  far  exceeds  the  former. 
Although  boiler  A,  under  the  given  conditions,  evaporates  about  9  per  cent  more 
than  boiler  C,  boiler  C  is  the  more  economical  in  the  combustion  of  coal  by 
about  40  per  cent 

Table  No.  60.  —  Relative  Efficiency  of  Boilers. 


Designation  of  Boiler. 

Heat  Value  of  One  Pound 
of  Coal. 

Evaporation  from  and  at  212° 
per  Pound  of  Coal. 

Efficiency. 

B.  T.  U.                                       Pounds. 

Per  cent. 

A 

14,500 

7-5° 

5° 

B 

11,250 

8.19 

7O 

C 

9,500 

6.86 

7° 

It  is,  therefore,  evident  that  the  evaporation  from  and  at  212°,  although  a  very 
convenient  basis  of  comparison  for  fuels,  may  not  be  properly  applied  in  defin- 
ing the  efficiency  of  a  boiler.  It  is  thus  that  the  distinction  is  to  be  drawn 
between  the  efficiency  of  a  fuel  and  that  of  the  boiler  in  connection  with  which 
it  is  burned.  A  more  accurate  basis  for  the  comparison  of  efficiency  of  differ- 
ent types  of  boilers  is  established  when  the  evaporation  is  expressed  in  pounds 
of  water  evaporated  per  pound  of  combustible.  But  even  this  is  somewhat 
affected  by  the  proportion  of  ash  and  elementary  water  in  the  coal. 

The  ideal  basis  of  comparison  is  to  be  sought  in  the  ratio  found  by  experi- 
ment to  exist  between  the  total  effective  heat  of  the  coal,  as  determined  by 
means  of  a  calorimeter,  and  that  rendered  evident  in  the  steam  generated,  which 
may  be  thus  expressed  :  — 

Efficiency  =  geat  units  usefully  applied^ 
Heat  units  supplied  to  furnace. 

Although  ideally  the  correct  method  of  comparing  the  efficiency  of  boilers,  and 
likewise  of  fuels,  when  proper  allowances  can  be  made  for  boiler  differences,  it 
is  practically  open  to  criticism  because  of  the  difficulty  in  determining  experi- 
mentally from  a  small  specimen  the  exact  heating  value  of  the  entire  quantity  of 
coal.  Even  with  this  basis  of  comparison,  relative  efficiency  tests  should  be 
conducted  under  identical  conditions  so  far  as  they  may  be  obtainable.  The 
complete  results  can  be  best  presented  in  the  form  of  a  heat  balance,  as 
follows  :  — 


MECHANICAL   DRAFT. 


97 


Heat  Balance. 


Dr. 

To  heat 
from  coal, 
from  air, 
from  feed-water. 


Cr. 


By  heat 

in  dry  steam, 

in  moisture  and  water  mechanically  suspended 

in  steam, 
in  dry  flue  gases, 

in  moisture  in  coal,  \  at  temper- 

in  water  resulting  from  combustion,  >-    ature  of 
in  vapor  in  air,  )  flue  gases, 

lost  through  incomplete  combustion  to  CO, 
in  ashes, 
lost  by  radiation  and  otherwise  unaccounted  for. 

As  is  the  case  with  fuel,  so  is  it  with  a  boiler  :  the  efficiency  must  be  consid- 
ered commercially.  For  this  reason  a  limit  is  reached  considerably  short  of 
100  per  cent,  beyond  which  the  loss  in  interest,  depreciation  and  other  fixed 
charges  exceeds  the  gain  from  decreased  cost  of  fuel,  per  unit  of  evaporation, 
resulting  from  the  given  improvement. 

In  illustration  of  the  difference  in  efficiency  of  different  types  of  boilers, 
Table  No.  61  is  presented.  This  covers  the  results  of  86  tests  conducted  by 
Mr.  Wm.  H.  Bryan1  under  common  conditions  and  with  ordinary  fuel,  principally 
Illinois  coal.  Due  allowance  is  to  be  made  for  locality,  special  type  of  boiler 
and  kind  of  coal. 

Table  No.  61.  —  Efficiency  of  Different  Types  of  Boilers. 


Maximum. 

Minimum. 

Average. 

I 
Small  vertical,                                                           3 

46.10 

34.60 

41.60 

Large  vertical,                                                           3 

52-3° 

49-00 

50.00 

Large  vertical,  improved  setting,                           i 



One  trial  only. 

67.89 

Tubular  boilers,                                                      14 

60.17 

44.76 

51-53 

Tubular  boilers,  improved  settings,                    34 

76.38 

41.94 

58.87 

Water-tube  boilers,                                                13 

70.11 

49-37 

61.31 

Water-tube  boilers,  improved  setting,                18 

81.32 

49-3° 

67.52 

EFFICIENCY  IN  PER  CENT. 


i  Boiler  Efficiency,  Capacity,  and  Smokelessness,  with  Low-Grade  Fuels. 
A  paper  read  before  the  Engineers'  Club  of  St.  Louis,  Oct.  21,  1896. 


Wm.  H.  Bryan. 


MECHANICAL    DRAFT. 


Rating  of  Steam  Boilers.  —  As  originally  the  most  important  purpose  of  the 
steam  boiler  was  to  generate  steam  for  use  in  a  steam  engine,  it  became  custo- 
mary to  express  its  capacity  by  the  nominal  output  of  the  engine  which  it  sup- 
plied. That  is,  its  rating  was  expressed  in  horse-power  (a  term  with  which  the 
boiler  has  properly  nothing  to  do)  ;  a  horse-power  in  each  case  representing  the 
weight  of  steam  required  per  hour  to  enable  the  engine  to  perform  work  contin- 
uously at  the  rate  of  33,000  foot-pounds  per  minute.  The  first  standard,  fixed 
by  Watt,  and  based  upon  the  performance  of  the  engines  of  his  day,  was  one 
cubic  foot  of  water  (weighing  about  60  pounds)  evaporated  per  hour  from  212° 
per  horse-power. 

As  the  efficiency  of  the  steam  engine  has  been  improved,  the  amount  of  steam 
necessary  for  the  production  of  a  horse-power  has  been  gradually  decreased. 
The  attempt  was  made  to  keep  pace  with  engine  improvements  by  correspond- 
ingly reducing  the  amount  of  water  represented  by  a  horse-power.  But  the 
present  existence  of  engines,  in  great  variety  of  design  and  manner  of  opera- 
tion, as  compared  with  the  simple  type  in  the  days  of  Watt,  renders  impos- 
sible the  establishment  of  any  definite  standard  of  rating  which  shall  apply  to 
them  all.  This  cannot  be  more  clearly  evidenced  than  by  Table  No.  62,  which 

Table  No.  62.  —  Steam  per  Horse-Power  per    Hour  for  Steam   Engines  with    Different 
Ratios  of  Expansion. 


RATIO  OF  EXPANSION. 

Steam 

Tvoe  of 

j.  ype  01 
Engine. 

above 
Atmosphere 

3 

4 

5 

7 

3° 

40 

39 

40 

40 

42 

45 

45 

35 

34 

36 

36 

38 

40 

60 

30 

28 

27 

26 

30 

32 

Non-condensing, 

75 

28 

27 

26 

25 

27 

29 

90 

26 

25 

24 

23 

25 

27 

!°5 

25 

24 

23         22 

22 

21 

'35 

24 

23 

22          21 

20 

2O 

15 

3° 

28 

28 

30 

35 

40 

30 

28 

27 

27          26 

28 

32 

45 

27 

26 

25         24 

25 

27 

Condensing, 

60 

26 

25 

25         23 

22 

24 

75 

26 

24 

24         22 

21 

2O 

105 

25 

23 

23         22 

21 

2O 

135 

25 

23 

22          21 

20 

'9 

MECHANICAL    DRAFT.  99 

embodies  Prof.  R.  H.  ThurstonV  estimate  of  the  steam  consumption  of  the 
best  classes  of  engines  in  common  use  and  in  good  order.  Evidently  the  term 
"  horse-power,"  applied  to  the  rating  of  steam  boilers,  must,  therefore,  be  consid- 
ered as  a  standard  of  measurement  rather  than  a  direct  measure  of  capacity. 

In  1876  the  committee  of  judges  of  the  Centennial  Exhibition,  to  whom  was 
entrusted  the  trials  of  the  competing  boilers  exhibited,  decided  to  adopt  a 
standard  rating  upon  conditions  considered  by  them  to  represent  fairly  average 
practice.  The  standard  unit  of  30  pounds  of  water  of  100°  temperature  evapo- 
rated into  dry  steam  of  70  pounds  gauge  pressure,  thus  adopted,  has  since 
become  the  almost  universal  standard  of  rating  in  the  United  States  for  the 
nominal  evaporative  capacity  of  steam  boilers,  and  is  commonly  designated  as 
a  commercial  horse-power.  The  amount  of  heat  required  to  evaporate  one 
pound  of  water  under  these  conditions  is  1,110.2  B.  T.  U.,  equivalent  to  1.1496 
units  of  evaporation,  as  previously  denned.  An  evaporation  of  30  pounds 
under  the  stated  condition  is,  therefore,  equivalent  to  the  development  of  33,305 
B.  T.  U.  per  hour,  or  34.488  pounds  of  water  evaporated  from  and  at  212°,  or  in 
round  numbers  34.5  pounds.  Although  even  this  rating  is  now  open  to  criti- 
cism, because  it  does  not  represent  the  present  standard  of  average  steam-engine 
performance,  nevertheless,  being  a  more  or  less  arbitrary  standard,  it  would 
appear  to  be  as  satisfactory  as  any  other  for  the  mere  purpose  of  comparing 
the  capacity  of  different  boilers.  Any  standard  of  this  character  is,  however, 
open  to  the  criticism  that  because  of  the  range  of  capacity  possessed  by  any 
boiler  it  is  difficult  to  fix  the  conditions  under  which  this  capacity  should  be 
attained.  Should  they  be  those  that  exist  when  the  boiler  is  being  run  easily 
under  ordinary  conditions,  or  should  the  measure  of  capacity  be  taken  when 
the  boiler  is  pushed  to  its  utmost  ?  The  above-mentioned  committee  considered 
that  a  boiler  should  be  capable  of  developing  its  stipulated  power  with  easy 
firing,  moderate  draft  and  ordinary  fuel,  while  exhibiting  good  economy ;  and 
that  the  boiler  should,  furthermore,  be  capable  of  developing  at  least  one-third 
more  than  its  rated  power  to  meet  emergencies.  It  is  obvious,  nevertheless, 
that  no  matter  what  the  basis  adopted,  a  rating  based  upon  the  horse-power 
standard  possesses  considerable  elasticity. 

Table  No.  63  is  presented  for  the  purpose  of  simplifying  the  reduction  of 
horse-power  under  any  given  conditions  to  that  under  other  conditions. 

The  basis  is  30  pounds  of  water  at  100°  evaporated  into  steam  of  70  pounds 
pressure.  At  any  other  temperature,  as  for  instance  170°,  and  pressure  of  say 
80  pounds,  the  equivalent  evaporation  is  shown  to  be  31.96  pounds. 


i  A  Manual  of  Steam  Boilers.     R.  H.  Thurston.     New  York, 


MECHANICAL    DRAFT. 


Table  No.  63.  —  Required  Hourly  Evaporation  per  Commercial  Horse-Power  at  Various  Temperatures  of  Feed  and 
Pressure  of  Steam. 

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Temperature 
;>f  Keed-Water. 

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8,8 

MECHANICAL   DRAFT.  101 

The  preceding  applies  most  directly  to  the  rating  of  boilers  as  determined  by 
experimental  test.  But  it  is  obviously  desirable  that,  for  the  purposes  of  desig- 
nation, the  capacity  or  rating  of  a  given  design  of  boiler  should  be  at  least 
approximately  known  before  it  is  constructed.  Evidently  such  a  rating  must  be 
based  upon  previous  experiment  with  similar  boilers,  having  the  same  general 
proportions  and  disposition  of  heating  surface  and  operating  under  similar  con- 
ditions. The  measure  of  such  results  is  made  in  the  number  of  pounds  of  water 
evaporated  per  hour  per  square  foot  of  heating  surface,  or  its  equivalent,  the 
number  of  square  feet  of  heating  surface  per  commercial  horse-power.  In 
boilers  of  the  same  relative  proportions  of  grate  to  heating  surface,  and  of  the 
same  general  arrangement  of  heating  surface,  the  rate  of  evaporation  is  fairly 
constant  for  ordinary  draft  conditions.  That  is,  the  total  evaporation  is  prac- 
tically proportional  to  the  heating  surface.  The  area  of  this  surface,  therefore, 
forms  a  ready  means  of  comparison  of  capacity.  But  the  location  and  character 
of  the  heating  surface  largely  determines  the  rate  of  evaporation,  thus  restrict- 
ing direct  comparison  to  boilers  of  the  same  type  under  the  same  conditions. 
A  fair  average  for  boilers  of  the  ordinary  horizontal  return  tubular  type,  with 
ordinary  natural  draft,  is  an  evaporation  of  3  pounds  of  water  from  and  at  212° 
per  square  foot  of  heating  surface.  On  the  basis  of  34.5  pounds  per  horse- 
power, under  these  conditions,  this  is  equivalent  to  one  horse-power  for  each 
11.5  square  feet  of  heating  surface.  The  number  of  square  feet  of  heating  sur- 
face per  horse-power,  for  other  rates  of  evaporation,  is  presented  in  Table  No.  64. 

Table  No.  64. —  Square  Feet  of  Heating  Surface  per  Horse-Power. 


Pounds    of    water    evaporated! 

1 

from  and  at  212°  per  square  \ 
foot  of   heating  surface  per  i 

2.0 

2-5 

3-° 

3-5 

4.0 

5-° 

6.0 

7.0 

8.0 

9.0      10. 

hour, 

i 

Square  feet  of  heating  surface  \ 
required  per  horse-power,         ' 

17-3 

13.8 

"•5 

9.8 

8.6 

6.8 

5-8 

4-9 

4-3 

3-8     3-5 

Under  mechanical  draft  the  rate  of  evaporation  per  unit  of  heating  surface 
is  greatly  increased.  Thus  "the  maximum  power  obtained  with  forced  draft 
and  an  air  pressure  not  exceeding  2  inches  of  water,"  as  stated  by  Sir  W.  H. 
White,1  "has  varied  from  40  to  50  per  cent  increase  above  the  maximum  power 


i  On  the  Speed  Trials  of  Recent  Warships.  Sir  W.  H.  White.  Transactions  of  Institution  of 

Naval  Architects.     1886. 


MECHANICAL   DRAFT. 


obtained  by  natural  draft."  The  obvious  conclusion  is  that  a  smaller  boiler 
will,  under  forced  draft,  do  the  same  work.  In  fact,  forced  draft  has  been  gen- 
erally applied,  particularly  in  the  marine  service,  "  with  a  view  to  lessening  the 
dimensions  of  the  boiler.  This  is  rendered  possible  by  the  increased  tempera- 
ture of  the  fire  being  able  to  transmit  more  heat  per  unit  of  surface,  and  to 
render  the  boiler,  surface  for  surface,  more  efficient  than  it  would  be  if  a  lower 
fire  temperature  were  used.  It  is  evident  that  if  the  boiler  surfaces  are  more 
efficient,  less  heat  will  be  wasted  up  the  chimney."' 

The  horse-power  of  various  types  of  boilers,  based  upon  the  area  of  heating 
surface  shown  by  experiment  to  be  necessary  for  the  evaporation  of  34.5  pounds 
of  water  from  and  at  212°  per  hour,  is  usually  accepted  as  the  nominal  rating; 
owing  to  slight  differences  even  between  boilers  of  the  same  type,  there  is  some 
latitude  in  this  basis  of  measurement.  The  range  of  proportions  generally 
adopted  in  practice  is  presented  in  Table  No.  65. 

Table  No.  65.  —  Approximate  Relation  of  Horse-Power  and  Heating  Surface  in 
Different  Types  of  Boilers. 


ii 

III 

CO  «  »• 

e* 

ill 

TYPE  OF  BOILER. 

ijj 

1§ 

111 

Authority. 

f 

il 

I 

*a« 

Water-tube, 

10  tO  12 

°-3 

1.  00 

1.  00 

Isherwood. 

Tubular, 

I4toi8 

0.25 

0.91 

0.50 

Isherwood. 

Flue, 

8  to  12 

0.4 

0.79 

0.25 

Prof.  Trowbridge. 

Plain  cylinder, 

6  to  10 

0.5 

0.69 

O.2O 

Locomotive, 

1  2  to  1  6 

0.275         0.85 

o-55 

Vertical  tubular, 

1  5  to  20 

0.25 

0.80 

0.60 

Radiation  and  Convection  of  Furnace  Heat.  —  The  ideal  temperature  of  com- 
bustion of  a  coal  consisting  of  80  per  cent  carbon  has  already  been  shown  by 
Table  No.  20  to  be  4,718°  when  the  chemically  necessary  amount  of  air  is  sup- 
plied. If  this  supply  be  doubled,  the  temperature  is  reduced  to  2,600°;  in 
each  case  the  temperature  being  measured  above  that  of  the  air  supplied  to  the 
furnace.  These  ideal  temperatures  pertain  to  the  heart  of  the  fire  and  can  only 
exist  in  the  furnace  chamber  if  the  fire  is  properly  enclosed  with  radiating 
surface. 

The  ordinary  boiler  plate,  with  the  hot  gases  on  one  side  and  water  on  the 


1  On  the  Progress  and  Development  of  Marine  Engineering.     Wm.  Parker.     Transactions  of 
Institution  of  Naval  Architects.     1887. 


MECHANICAL   DRAFT. 


103 


other,  presents  a  very  different  condition,  for  it  becomes  a  greedy  absorber  of 
heat,  both  radiant  and  convected.  The  air  and  gases,  being  poor  conductors 
of  heat,  and  absorbing  but  a  very  slight  amount  of  the  radiant  heat,  have  only 
the  power  to  increase  the  temperature  of  the  surface  with  which  they  come  in 
contact  by  the  process  of  convection  or  carrying,  which  may  be  defined  as 
"  a  transfer  and  diffusion  of  the  state  of  heat  in  a  fluid  mass  by  means  of  the 
motion  of  the  particles  of  that  mass."1 

The  heat  that  is  radiated  from  the  fire  is  but  feebly  reciprocated  from  the 
plate  surfaces  of  the  boiler,  since  the  plate  is  maintained  at  a  temperature  not 
much  higher  than  that  of  the  water  inside.  Under  such  conditions  the  heat 
which  is  radiated  from  the  fuel  upon  the  grate,  together  with  that  which  is  com- 
municated by  convection  from  the  heated  gases,  is  rapidly  absorbed  and  carried 
off.  It  is,  therefore,  impossible  to  maintain  in  the  furnace  a  temperature  even 
near  the  maximum  temperature  of  combustion.  The  radiation  from  the  fuel, 
taking  place,  as  it  does,  in  straight  lines,  is  thereby  restricted  in  its  effect  to  the 
grates,  walls  of  the  furnace  chamber  and  the  exposed  portion  of  the  boiler. 
The  heat  not  thus  lost  is  carried  along  by  the  products  of  combustion. 

The  exact  proportional  relation  between  the  radiant  and  convected  heat  is 
difficult  of  determination,  but  Peclet  assumed  them  to  be  equal  as  they  leave 
the  upper  surface  of  the  fuel.  Upon  this  assumption  and  the  formula  derived 
by  Dulong  and  Petit,  the  relation  of  radiant  and  convected  heat  has  been  cal- 
culated by  D.  K.  Clark2  for  different  rates  of  combustion.  The  conditions 
imposed  are :  complete  combustion,  no  excess  of  air  and  a  coal  having  a  heat 
value  of  14,700  B.  T.  U.  The  results  are  given  in  Table  No.  66,  with  the  aver- 
age relative  proportions  existing  between  the  two  means  of  dissipation  of  heat. 

Table  No.  66.  —  Temperature  and  Heat  of  a  Coal  Fire  in  a  State  of  Incandescence. 


Coal 
Consumed  per 
Square  Foot 
of  Grate  per 

Temperature 
of  Surface 
of  Boiler 

Temperature 
of  Surface 

Approximately  Calculated  Distribution  of  the  Heat  of  Combustion. 

By  Radiation. 

By  Convection  of  Gases. 

Sum  of 
Radiation  and 

Hour. 
Pounds. 

Degrees  Fahr. 

Degrees  Fahr. 

B.  T.  U. 

Per  Cent 
of  Sum. 

B.  T.  U. 

Per  Cent 
of  Sum. 

Convection. 
B.  T.  U. 

5 

350° 

1,4000 

53>96o 

74 

19,160 

26 

73,120 

10 

35° 

1>SS° 

102,500              70 

43,5!° 

3° 

146,010 

20 

35° 

!,705 

198,400              67 

96,080 

33 

294,480 

40 

35° 

1,857 

378,650              64                209,850 

36 

588,500 

80 

35° 

2,009              721,800           61 

455'400 

39 

1,177,200 

1  20 

35° 

2,097            1,049,000           59        1     714,050 

4i 

1,763,050 

1  A  Manual  of  the  Steam  Engine.     W.  J.  M.  Rankine.     London,  1885. 

2  The  Steam  Engine.     D.K.Clark.     London,  &c.,  1890. 


104 


MECHANICAL   DRAFT. 


Distribution  of  the  Heat  of  Combustion.  —  It  being  the  function  of  a  boiler 
to  utilize  as  much  as  possible  of  the  heat  generated  by  the  combustion  of  the 
fuel  in  the  furnace  provided  for  the  purpose,  it  is  to  be  expected  that  the  gases 
will  be  gradually  cooled  as  they  approach  the  chimney.  The  greater  the 
decrease,  and,  other  things  equal,  the  lower  the  final  temperature,  the  higher  the 
efficiency.  The  temperature  of  the  gases  at  different  points  in  their  passage 
across  the  heating  surface  of  the  boiler  obviously  depends  upon  the  character, 
extent  and  temperature  of  that  surface,  and  the  initial  temperature  and  velocity 
of  the  gases. 

The  absorption  of  practically  all  of  the  radiant  and  part  of  the  convected 
heat  by  the  surfaces  exposed  to  the  fire  immediately  lowers  the  temperature  of 
the  gases  so  that  as  they  pass  onward  to  the  other  portions  of  the  heating  sur- 
face their  heating  power  is  much  decreased.  The  existence  of  this  condition 
is  rendered  evident  by  the  results  of  tests  reported  by  Mr.  Paul  Havrez.1 
These  tests  were  conducted  upon  a  special  boiler  of  the  locomotive  type,  the 
barrel  or  cylindrical  portion  of  which  was  divided  into  four  sections  of  equal 
length,  and  so  arranged  that  the  evaporation  in  each  could  be  ascertained  sep- 
arately. The  general  proportions  of  this  boiler,  together  with  the  results,  are 
presented  in  Table  No.  67,  from  which  it  is  evident  that  the  surface  of  the  fire- 
Table  No.  67.  —  Evaporation  in  Different  Sections  of  Experimental  Boiler. 


Pounds  of  Water  Evaporated  per  Hour  per  Square  Foot  of 
Heating  Surface. 

Portion  of  Boiler. 

Square  Feet 
of  Heating  Surface. 

With  Coke  for  Fuel. 

With  Briquettes  for  Fuel. 

Firebox, 

76-43 

24.5 

36.9 

First  section, 

I79 

8.72 

11.44 

Second  section, 

179 

4.42 

5-72 

Third  section, 

I79 

2.52 

3-52 

Fourth  section, 

179 

1.68 

2.31 

box  was  about  three  times  as  efficient  as  that  of  the  first  section,  and  increas- 
ingly more  for  the  other  sections  respectively. 

The  variation  in  the  rate  of  absorption  of  heat  from  the  gases,  which  takes 
place  in  their  passage  through  the  tubes  of  a  marine  boiler,  is  clearly  shown  by 
the  results  of  tests  conducted  under  the  supervision  of  Mr.  A.  J.  Durston,  Engi- 


Proceedings  of  Institute  of  Civil  Engineers.     London,  1875. 


MECHANICAL    DRAFT. 


IC5 


neer  in  Chief  of  the  British  Navy,  as  presented  in  Table  No.  68.  These  results 
cover  eight  sets  of  records  of  temperatures  taken  by  a  Le  Chatelier  thermo- 
electric pyrometer  inserted  to  various  distances  in  the  tubes.  The  boiler  was 
being  worked  at  its  normal  capacity,  the  rate  of  consumption  of  coal  being 
about  17  pounds  per  square  foot  of  grate.  The  temperature  existing  in  the 
combustion  chamber  was  1,644°,  ar>d  that  just  inside  the  tube  1,550°. 


Table  No.  68. —  Temperatures  in  Tubes  of  Marine  Boiler. 


LOCATION. 

Temper- 
ature. 

LOCATION. 

Temper- 
ature. 

i  inch  from  combustion  chamber, 

1,4660 

i  ft.  2  in.  from  combustion  chamber, 

1,368° 

2  inches  from  combustion  chamber, 

1,426 

i  ft.  8  in.  from  combustion  chamber, 

1,^95 

3  inches  from  combustion  chamber, 

1,405 

2  ft.  8  in.  from  combustion  chamber, 

1,198 

4  inches  from  combustion  chamber, 

1,412 

3  ft.  8  in.  from  combustion  chamber, 

1,  1  06 

5  inches  from  combustion  chamber, 

1,398 

4  ft.  8  in.  from  combustion  chamber, 

1,015 

6  inches  from  combustion  chamber, 

1,406 

5  ft.  8  in.  from  combustion  chamber, 

926 

7  inches  from  combustion  chamber, 

1,400 

6  ft.  8  in.  from  combustion  chamber, 

887 

8  inches  from  combustion  chamber, 

1,410 

In  smoke  box, 

782 

The  temperatures  existing  under  usual  conditions,  in  connection  with  an  ordi- 
nary horizontal  return  tubular  boiler,  were  very  carefully  ascertained  by  Mr. 
J.  C.  Hoadley,1  and  are  presented  in  Table  No.  69.  The  coal  burned  consisted 
of  82  per  cent  carbon  completely  consumed  to  carbonic  acid,  the  steam  pres- 
sure was  about  45  pounds  above  the  atmosphere  and  the  air  supply  per  pound 
of  fuel  was  21.28  pounds.  The  actual  temperature  shown  to  exist,  as  compared 
with  that  which  the  coal  should  be  ideally  capable  of  producing,  with  no  excess 
of  air,  as  previously  indicated,  is  particularly  to  be  noted. 

Table  No.  69.  —  Temperatures    in  Connection  with    Horizontal  Return  Tubular  Boiler. 


Location  at  which  Temperature  was  Taken. 


Temperature. 


In  heart  of  fire    . 

At  bridge  wall 

In  smoke  box 

Air  admitted  to  furnace 

Steam  and  water  in  boiler    . 

Gases  escaping  to  chimney  . 


2,4260 

i,34i 

368 

78 

292 


Warm-Blast  Steam-Boiler  Furnace.     J.  C.  Hoadley.     New  York,  1886. 


io6  MECHANICAL   DRAFT. 

Disposition  of  Heat  in  Steam  Boilers.  —  The  theoretical  heat  losses  incident 
to  the  combustion  of  a  given  amount  of  coal  have  already  been  considered  in 
the  preceding  chapter.  These  indicate  clearly  the  disposition  of  all  the  heat 
generated,  with  the  exception  of  that  lost  through  the  brickwork.  This  loss  is 
evidently  variable  and  uncertain,  but  under  ordinary  conditions  is  between  4 
and  20  per  cent.  Mr.  J.  C.  Hoadley1  conducted  a  series  of  experiments  upon 
the  setting  of  a  horizontal  return  tubular  boiler,  by  inserting  thermometers  to 
different  depths  in  holes  in  the  .walls  at  5  feet  above  the  floor,  —  opposite  to  the 
body  of  the  boiler  and  midway  between  the  bridge  wall  and  the  pier.  The 
maximum  and  minimum  temperatures  found  between  8.30  a.m.  and  4.30  p.m., 
at  different  depths,  upon  one  day  of  the  test,  are  presented  in  Table  No.  70. 
Of  course  the  temperature  at  the  outer  surface  of  the  setting  would  be  consider- 
ably less  than  that  4  inches  inward,  owing  to  the  rapid  radiation  at  the  surface. 

Table  No,  70.  —  Temperatures  of   Brick  Setting  of   Horizontal  Return  Tubular  Boiler. 


Place  of  Observation. 

Minimum 
Temperature. 

Maximum 
Temperature. 

.    4  inches  from  outside  of  wall  

1400 

1820 

1  6  inches  from  outside  of  wall          .... 

285 

353 

28  inches  from  outside  of  wall          .... 

297 

460 

The  disposition  of  the  heat  generated  in  the  furnace  of  a  steam  boiler,  as 
determined  by  several  investigators  for  different  types  of  boilers,  is  indicated  in 
Table  No.  71.  The  boiler  experimented  on  by  Dr.  Bunte  was  of  ordinary 
character;  that  designated  A  and  reported  upon  by  MM.  Scheurer  and  Meunier 
was  of  the  French  type,  having  three  heaters  with  six  feed-heater  tubes  at  one 
side ;  while  the  results  given  under  B  are  the  average  of  experiments  upon  four 
different  types  of  boilers.  Boiler  C,  experimented  upon  by  Messrs.  Donkin  and 
Kennedy,  was  of  vertical  tubular  construction  with  an  internal  firebox ;  boiler 
D  was  of  the  locomotive  pattern  ;  while  E  was  of  "elephant"  type,  provided  with 
Green's  economizer.  The  boiler  tested  by  Hoadley  was  of  the  ordinary  multi- 
tubular  form,  provided  with  a  special  air-heating  arrangement  which  lowered  the 
temperature  of  the  flue  gases  about  213°,  and  raised  that  of  the  air  supplied  to 
the  furnace  about  300°.  These  abstractors  account  for  the  high  efficiency 
secured,  which  might  have  been  closely  approached  by  boiler  E  had  it  not  been 
for  the  excessive  loss  through  the  brickwork. 


Warm-Blast  Steam-Boiler  Furnace.     J.  C.  Hoadley.     New  York,  1886 


MECHANICAL  DRAFT. 


107 


All  figures  are  given  in  percentages  of  the  total  amount  of  heat  accounted  for, 
The  percentage  of  heat,  which  is  indicated  to  have  been  disposed  of  in  heating 
and  evaporating  the  water,  is  a  direct  measure  of  the  efficiency  of  the  boiler. 

Table  No.  71.  —  Disposition  of  Heat  in  Steam  Boilers. 


AUTHORITY. 

DISPOSITION  OF  HEAT. 

Scheurer  and 
Meunier. 

Donkin  and  Kennedy. 

I 

Bunt<5. 

A 

B 

C 

D 

E 

K 

Waste  in  flue  gases  including  evaporation  ~) 

of  moisture  in  coal  and  heating  vapor  1 
in  air  when  these  losses  are  not  sepa-  j 

1  8.6 

5-5 

14.8 

9-4 

22.5 

6.5 

5-04 

rately  given, 

Evaporating  moisture  in  coal, 

3-5 

2-5 

6.1 

O.I 

O.I 

0.0 

'•55 

Heating  vapor  in  air, 

o.iS 

Imperfect  combustion, 

8.0 

6.0 

12.7 

o.o 

0.0 

1.44 

Clinker  and  ash, 

4.1 

O.I 

O.2 

0.0 

Radiation    and    heat    not     otherwise    ac-  \ 
counted  for,                                                j 

7.6 

23-5 

13-4 

13-9 

1  1.0 

15.0 

4.00 

Heating  and  evaporation  of  water, 

58.2 

61.0 

65.7 

63.8 

66.2 

78.5 

87.79 

Sources  of  Efficiency.  —  With  a  given  fuel,  and  otherwise  identical  conditions, 
the  efficiency  of  a  boiler  is  most  largely  dependent  upon  the  relation  of  its  grate 
area  to  its  heating  surface,  and  upon  the  rate  of  combustion  of  the  fuel.  Under 
the  same  conditions  of  boiler  and  fuel,  the  greater  the  quantity  consumed  per 
hour  the  greater  is  the  amount  of  water  evaporated  per  hour.  But  at  the  same 
time  the  quantity  of  water  evaporated  per  pound  of  fuel  decreases  because  of 
the  higher  temperature  of  the  escaping  gases.  This  loss  can  only  be  diminished 
by  increasing  the  heating  surface  either  in  the  boiler  or  in  a  separate  heater, 
while  the  decreased  draft,  due  to  lowered  temperature,  is  easily  made  good  by 
mechanical  means.  It  is  to  be  carefully  noted  that  all  this  relates  to  a  condi- 
tion of  increased  boiler  capacity,  resulting  from  a  greater  coal  consumption. 
But  when  the  capacity  and  coal  consumption  are  maintained  practically  con- 
stant, a  higher  efficiency  per  pound  of  fuel  may  actually  be  obtained  by  a  mod- 
erate reduction  of  grate  area,  whereby  the  surface  ratio  is  increased,  and  a 
corresponding  increase  in  the  rate  of  combustion  per  square  foot  of  grate.  This 
matter  of  the  relation  between  the  surface  ratio  and  the  rate  of  combustion  and 


io8  MECHANICAL    DRAFT. 

its  economical  influence,  as  indicated  in  the  evaporation  per  unit  of  fuel,  will  be 
discussed  at  considerable  length  in  the  succeeding  chapter. 

The  quality  of  the  fuel  must  to  a  certain  extent  enter  into  the  problem  of 
boiler  design  to  secure  the  highest  efficiency.  The  desirable  features  to  give 
the  best  results  with  low-grade  fuels  are  concisely  stated  by  Mr.  W.  H.  Bryan1 
to  be  — 

"A.  Ample  draft;  i  inch  of  water  or  even  more.  Good  results  cannot  be 
secured  with  drafts  less  than  one-half  inch.  Good  draft  and  thick  beds  of  fuel 
permit  high  firebox  temperatures,  which  we  have  found  absolutely  necessary. 

"  B.  Large  ratio  of  heating  to  grate  surface,  so  that  while  burning  coal  at  a 
high  rate  per  square  foot  of  grate  per  hour,  there  is  sufficient  heating  surface  to 
reduce  the  temperature  of  the  flue  gases  to  450°  Fahr.  or  less. 

"  C.  The  combustion  chamber  should,  if  possible,  be  separate  from  the  heat- 
ing surfaces,  so  as  to  avoid  their  cooling  effect.  It  should  be  quite  deep  —  30 
inches  or  more." 

As  indicating  the  importance  of  the  draft,  and  the  practical  necessity  of 
mechanical  means  for  creating  the  same,  Mr.  Bryan  states  :  "  To  secure  the  very 
highest  results,  the  gases,  after  leaving  the  boiler-heating  surfaces  at  not  exceed- 
ing 500°,  should  be  passed  through  feed-water  economizers  and  thence  through 
air  heaters.  The  feed-water,  leaving  the  ordinary  exhaust  heater  at  a  little 
above  200°  Fahr.,  may  be  raised  to  over  300°  in  the  economizer,  and  the  heated 
gases  reduced  to  250°,  or  less.  This  reduction  in  temperature,  of  course, 
destroys  the  usefulness  of  these  gases  as  draft  producers,  unless  the  chimney  is 
very  tall.  The  draft,  however,  can  be  better  produced  by  exhaust  fans,  which 
draw  the  air  through  and  out  of  the  furnace  and  economizer,  and  discharge  the 
gases  at  such  a  height  above  the  roof  that  they  will  not  be  objectionable ;  thus 
doing  away  entirely  with  the  necessity  for  high  chimneys.  Still  better  economy 
may  be  secured  by  placing  air  heaters  in  the  smoke  flue,  beyond  the  fan  or 
between  it  and  the  economizer.  Through  this  the  air,  entering  the  ashpit  for  pur- 
poses of  combustion,  may  be  drawn,  so  that  the  heated  gases  are  finally  dis- 
charged at  a  temperature  but  little  above  that  of  the  atmosphere.  The  speed  of 
the  fan  may  be  controlled  by  an  automatic  regulator,  which  increases  the  speed 
of  the  fan  engine  as  the  steam  pressure  drops,  and  reduces  it  as  the  pressure 
increases ;  thus  performing  all  the  functions  of  an  automatic  damper  regulator. 
This  plan  is  not  experimental  or  untried,  but  has  already  been  adopted  in 
numerous  large  plants." 


'  Boiler  Efficiency,  Capacity  and  Smokelessness,  with  Low-Grade  Fuels.      Wm.  H.  Bryan. 
A  paper  read  before  the  Engineers'  Club  of  St.  Louis,  Oct.  21,  1896. 


MECHANICAL   DRAFT.  109 

The  influence  of  the  relation  between  area  of  grate  and  of  heating  surface 
was  very  carefully  investigated  by  Mr.  D.  K.  Clark,1  in  the  case  of  a  locomotive 
boiler  using  coke.  From  these  tests  he  deduced, — 

"  First.  That,  assuming  throughout  a  constant  efficiency  of  the  fuel  or  pro- 
portion of  water  evaporated  to  the  fuel,  the  evaporative  performance  of  a  loco- 
motive boiler,  or  the  quantity  of  water  which  it  is  capable  of  evaporating  per 
hour,  decreases  directly  as  the  grate  area  is  increased;  that  is  to  say,  the  larger 
the  grate  the  smaller  is  the  evaporation  of  water  when  the  efficiency  of  the  fuel 
is  the  same,  even  with  the  same  heating  surface. 

"  Second.  That  the  evaporative  performance  increases  directly  as  the  square  of 
the  heating  surface,  with  the  same  area  of  grate  and  efficiency  of  fuel. 

"  Third.  The  necessary  heating  surface  increases  directly  as  the  square  root 
of  the  performance ;  that  is  to  say,  for  example,  for  four  times  the  performance, 
with  the  same  efficiency,  twice  the  heating  surface  only  is  required. 

"  Fourth.  The  necessary  heating  surface  increases  directly  as  the  square  root 
of  the  grate,  with  the  same  efficiency ;  that  is  to  say,  for  instance,  if  the  grate 
be  enlarged  to  four  times  its  first  area,  twice  the  heating  surface  would  be 
required,  and  would  be  sufficient  to  evaporate  the  same  quantity  of  water  per 
hour  with  the  same  efficiency  of  fuel." 

The  relation  between  the  area  of  grate  and  of  heating  surface,  which  has 
already  been  expressed  as  the  "surface  ratio,"  may  be  thus  represented:  — 

Surface  ratio  =  Area  of  heating  surface. 
Area  of  grate  surface. 

This  ratio  naturally  varies  according  to  the  type  of  boiler,  the  general  prac- 
tice being  about  as  indicated  in  Table  No.  72. 

Table  No.  72. —  Surface  Ratios  of  Steam  Boilers. 


TYPE  OF  BOILER. 


Area  of  Heating  Surface 
when  Grate  Area=  i. 


Marine  return  tubular 

Lancashire 

Cornish  .         .         .         . 

Modified  locomotive  type 

Horizontal  return  tubular 


25  to  38 

26  to  33 
25  to  40 
30  to  34 
30  to  50 


Water-tube      .         .         . 35  to  65 


Horizontal  internally  fired  multi-tubular 
Locomotive  .  .  .  '  .  ... 
Plain  cylinder  .  .  • .  .  .  . 


25  to  45 
60  to  90 
10  to  15 


The  Steam  Engine.     U.  K.  Clark.     London,  «ic.,  1890. 


no  MECHANICAL    DRAFT. 

As  it  has  already  been  shown, —  that  when  the  total  coal  consumption  is 
increased  the  heating  surface  must  also  be  increased  to  maintain  the  same 
efficiency, —  so  the  converse  must  be  evident :  that  with  a  given  rate  of  combus- 
tion it  is  not  economical  to  increase  the  area  of  heating  surface  beyond  certain 
limits.  These  limits  must  of  necessity  be  determined  by  experiment. 

Mr.  D.  K.  Clark1  summarized  his  deductions  from  a  large  number  of  tests  of 
boilers  of  different  types  in  the  following  formulae  :  — 

Stationary  boilers      .         .         .  w  =  0.0222  r2  -f-    9.56^. 

Marine  boilers  .         .         .  w  =  0.016  r2    -f"  IO-25  c- 

Portable  engine  boilers     .         .  w  =  0.008  r2    -f-    8.6  c. 

Locomotive  boilers  (coal-burning),  w=  0.009  r2    ~\~    9-7  c- 

Locomotive  boilers  (coke-burning),  w  =  0.0178  r2-\-    7.94*-. 

In  which  w  =  weight  of  water  in  pounds  per  square  foot  of  grate  per  hour. 
c  =  pounds  of  fuel  per  square  foot  of  grate  per  hour. 
r  =  ratio  of  heating  to  grate  surface. 

The  water  is  taken  as  evaporated  from  and  at  212°. 

The  ratio  of  grate  surface  to  heating  surface  being  one  of  the  factors,  it  is 
evident  that  by  means  of  these  formulae  the  evaporation  per  square  foot  of 
heating  surface  may  also  be  obtained.  There  are  minimum  rates  of  consump- 
tion of  fuel  below  which  these  formulas  are  not  applicable.  The  limit  varies 
for  each  kind  of  boiler  and  with  the  surface  ratio.  It  is  imposed  by  the  fact 
that  the  maximum  evaporative  power  of  fuel  is  a  fixed  quantity,  and  is  naturally 
at  that  point  where  the  reduction  of  the  rate  of  combustion  for  a  given  ratio 
procures  the  absorption  into  the  boiler  of  the  whole  of  the  proportion  of  the 
heat  which  is  available  for  evaporation.  In  the  combustion  of  good  coal  the 
limit  of  evaporative  efficiency  may  be  taken  as  measured  by  12.5  pounds  of 
water  from  and  at  212°.  Table  No.  73,  based  upon  these  formulae,  presents  the 
effect  of  increasing  rates  of  combustion  with  different  surface  ratios.  While 
for  a  given  boiler  and  surface  ratio  this  table  indicates  that  the  evaporative 
efficiency  decreases  as  the  rate  of  combustion  is  increased,  it  is  to  be  noted 
that  the  capacity  of  the  boiler  is  increased  also,  and  that  by  a  proper  applica- 
tion of  more  heating  surface  the  efficiency  may  be  maintained.  In  other  words, 
a  boiler  and  its  appurtenances  should  be  designed  for  a  given  rate  of  combus- 
tion. A  high  rate  of  combustion  is  not,  therefore,  an  indication  of  low  effi- 
ciency ;  but,  as  will  be  shown  later,  is,  with  a  proper  surface  ratio,  one  of  the 
important  factors  in  attaining  economy  in  boiler  practice. 


The  Steam  Engine.     D.  K.  Clark.     London,  &c.,  1890. 


MECHANICAL    DRAFT. 


Table  No.  73.  —  Evaporative  Performance  of  Steam  Boilers  for  Increasing  Rates  of 
Combustion  and  Different  Surface  Ratios  and  Best  Coal  and  Coke. 


Fuel  per  Square  Foot  of  Grate  per  Hour  in  Pounds. 

KIND  OF 

Water  from  and  at  212° 

BOILER. 

per  Hour. 

5 

10 

15 

20 

3° 

40          50 

j*>    Stationary, 

Per  square  foot  of  grate,    62.5* 

116 

163 

211 

3°7 

402 

498 

.2    Stationary, 

Per  pound  of  coal,                12.5       11.56 

10.89 

10.56 

10.23 

1  0.06 

9.96 

o    Marine, 

Per  square  foot  of  grate, 

62.5* 

"7 

1  68 

2I9 

322 

424 

527 

•c    Marine, 

j= 

Per  pound  of  coal, 

12.5 

11.69 

11.25 

10.95 

10.69 

io.6r 

10.54 

_.    Portable, 

Per  square  foot  of  grate, 

50.0         93 

136 

I79 

265 

35' 

437 

1    Portable, 

Per  pound  of  coal, 

i  o.o        9.3 

9.01 

8-95 

8.83 

8.77 

8.74 

fg    Locomotive, 

Per  square  foot  of  grate, 

57.0        105 

»54 

202 

299 

396 

493 

o    Locomotive,      Per  pound  of  coke, 

11.4 

10.5 

10.26 

IO.IO 

9-97 

9.90 

9.86 

Stationary, 

Per  square  foot  of  grate,    62.5*     125* 

187.5* 

247 

342 

438 

534 

Stationary, 

Per  pound  of  coal, 

12.5    !   12.5 

I2-5 

I2-33 

11.41 

10.95 

10.67 

jf    Marine, 

Per  square  foot  of  grate, 

62.5* 

125* 

187.5* 

245 

348 

450 

552 

•2    Marine, 

Per  pound  of  coal, 

12.5       12.5 

12.5 

12.25 

11.58 

11.25 

11.05 

CK 

g    Portable, 

Per  square  foot  of  grate, 

62.5* 

1  06 

149 

192 

278 

364 

45° 

|    Portable, 

Per  pound  of  coal, 

12.5 

10.6 

9-93 

9.6 

9.27 

9.10 

9.00 

GB 

Locomotive, 

Per  square  foot  of  grate, 

62.5* 

120 

1  68 

217 

3J4 

411 

508 

Locomotive, 

Per  pound  of  coke, 

12.5       11.95 

1 

11.20 

10.85 

10.45 

10.26 

10.15 

*  These  quantities  fall  below  the  scope  of  the  formulae  for  the  water,  as  explained  in  the  text. 

By  the  same  process  of  reasoning  employed  in  the  previous  chapter,  in  the 
discussion  of  the  influence  of  the  air  supply,  it  is  evident  that  the  higher 
temperature  of  the  escaping  gases  resulting  from  an  increased  total  coal  con- 
sumption is  due  to  the  increased  supply  of  air  and  the  higher  velocity  which, 
therefore,  ensues.  Although  this  higher  temperature  is  but  the  result  of  more 
rapid  combustion,  it  is  at  the  same  time  an  absolute  necessity  when  a  chimney 
alone  is  depended  upon  to  create  the  draft.  This  is  because  the  draft  required 
for  the  increased  combustion  and  air  supply  can  only  be  secured,  in  the  case  of 
a  chimney,  by  raising  the  temperature  of  the  chimney  gases.  Under  these  condi- 
tions any  attempt  to  reduce  this  final  temperature  by  the  addition  of  heating 
surface  must  of  necessity  tend  to  reduce  the  rate  of  combustion.  Furthermore, 
the  influence  upon  the  draft  of  such  additional  surface  will  be  twofold  :  it  will 


ii2  MECHANICAL    DRAFT. 

be  reduced  both  because  of  the  lower  temperature  in  the  chimney,  and  because 
of  the  increased  resistance  due  to  the  extended  surface.  The  rate  of  combus- 
tion can  only  be  maintained  by  supplementing  the  draft,  which  may  be  done 
by  introducing  a  fan  either  for  forcing  in  the  air  or  withdrawing  the  gases. 

But,  with  sufficient  draft,  higher  efficiency  may  evidently  be  secured  by 
increasing  the  heating  surface,  with  the  limitation  that  the  surface  shall  not  be 
so  great  as  to  cool  the  gases  too  near  to  the  temperature  of  the  steam  ;  for  it  is 
probable  that  there  can  be  no  active  transmission  of  heat  from  the  gases  with- 
out to  the  water  within  a  boiler,  with  less  than  75°  difference  of  temperature. 
Nevertheless,  one  of  the  vital  principles  underlying  the  attainment  of  economy 
in  the  generation  of  steam  is  a  low  temperature  of  the  escaping  gases. 

The  opportunity  for  securing  more  economic  results  by  reducing  the  temper- 
ature of  the  flue  gases  is  well  evidenced  in  the  results  of  seventeen  indepen- 
dent boiler  tests,  by  Messrs.  Donkin  and  Kennedy.  They  found  the  heat  lost 
up  the  stack,  where  no  economizer  was  used,  to  range  between  9.4  per  cent  and 
31.8  per  cent  of  the  total  heat  of  combustion,  the  average  being  20.3  per  cent. 
It  is,  therefore,  evident  that  in  this  direction  lies  one  of  the  greatest  oppor- 
tunities for  increasing  boiler  efficiency.  Although  additional  surface  may  be 
obtained  by  reducing  the  size  of  the  tubes  and  increasing  their  number,  or  by 
ribbing  them,  or  introducing  retarders,  it  is  usually  customary  to  abstract  the 
surplus  heat  from  the  gases  by  some  means  in  a  sense  independent  of  the 
boiler.  It  may  then  take  the  form  of  a  feed-water  heater,  otherwise  known  as 
an  economizer,  or  the  form  of  a  device  for  abstracting  the  heat  from  the  gases 
and  transferring  it  to  the  air  supplied  to  the  fuel,  or  both.  The  results  obtained 
by  either  of  these  methods  have,  in  the  case  of  chimney  draft,  always  been 
restricted  by  the  cost  of  the  excessively  high  chimney  necessary  to  produce  the 
requisite  draft  with  the  decreased  temperature  and  increased  resistance.  The 
simplicity  and  efficiency  of  mechanical  draft,  however,  obviates  this  difficulty, 
and  makes  possible  the  attainment  of  much  lower  final  temperatures  of  the  flue 
gases  with  a  corresponding  increase  of  efficiency. 

Flue  Feed-Water  Heaters  or  Economizers.  —  The  modern  type  of  fuel  econo- 
mizer consists  of  a  series  of  tubes,  made  up  in  sections,  connected  at  the  ends 
and  placed  in  a  brick  chamber,  through  which  the  gases  pass  from  the  boiler 
to  the  chimney  or  fan.  Feed-water  is  forced  through  the  tubes,  while  the  gases 
circulate  around  them.  The  difference  between  the  economizers  of  different 
makes  lies  principally  in  the  proportions,  the  design  of  end  connections,  and 
the  position  of  the  tubes.  As  is  evident  from  Table  No.  76,  and  the  accom- 
panying explanation,  no  economizer  is  complete  without  some  device  for  con- 
tinuously or  periodically  removing  the  soot  from  the  exterior  of  the  tubes. 


MECHANICAL   DRAFT. 


The  economy  resulting  from  the  introduction  of  an  economizer  when  the 
draft  is  sufficient  naturally  depends  upon  the  normal  temperature  of  the  flue 
gases  escaping  from  the  boilers,  and  of  the  feed-water  supplied  to  the  econo- 
mizer. The  percentage  of  gain  may  be  determined  by  the  following  formula :  — 

100  (T — /) 

Gam,  in  per  cent,  = =^ — 

H —  / 

In  which  T '=  heat  units  in  one  pound  of  feed-water  above  o°  after  heating; 
/  =  heat  units  in  one  pound  of  feed-water  above  o°  before  heating ; 
H=  heat  units  in  one  pound  of  steam  of  boiler  pressure  above  o°. 

Table  No.  74,  calculated  by  this  formula,  indicates  the  saving  under  different 
conditions  of  feed-water,  with  steam  of  70  pounds  boiler  pressure.  Of  course 

Table  No.  74.  —  Percentage  of  Saving  in  Fuel  by  Heating  Feed- Water. 
Steam  at  70  Pounds  Gauge  Pressure. 


Initial 
Temperature 

Feed-Water. 

' 

100° 

110° 

120° 

.30° 

140° 

150° 

160° 

170° 

1  80° 

190^ 

200° 

210° 

220° 

250° 

300° 

35° 

5-53 

6.38 

7-24 

8.09 

8.95 

9.89 

10.66 

11.52 

12.38 

13.24 

14.09 

14.95 

I5.8l 

19.40 

29-34 

40 

5.12 

5-97 

6.84 

7.69 

8.56 

9.42 

10.28 

11.14 

I2.OO 

12.87 

l3-73 

14.5915.45 

18.89 

28.78 

45 

4.71 

5-57 

6.44 

7-3° 

8.16 

9-03 

9.90 

10.76 

11.62 

12.49 

I3-36 

14.22 

15.09 

'8.37 

28.22 

50 

4-3° 

5.16 

6.03 

6.89 

7.76 

8.64 

9-5i 

10.38 

11.24 

I2.II 

12.98 

'3-85 

1472 

17.87 

27-67 

55 

3-89 

4-75 

5-63 

6.49 

7-37 

8.24 

9.11 

9.99 

10.85 

"•73 

12.60 

13.48 

f4-35 

17-38 

27.12 

60 

3-47 

4-34 

5.21 

6.08 

6.96 

7.84 

8.72 

9.60 

10-47 

u-34 

12.22 

13.10 

13.98 

1  6.86 

26.56 

65 

3-°5 

3-92 

4.80 

5-67 

6.56 

7-44 

8.32 

9.20 

10.08 

10.96 

11.84 

12.72 

13.60 

!6-35 

26.02 

70 

2.62 

3-5° 

4-38 

5.26 

6.15 

7-03 

7.92 

8.80 

9-68 

10.57 

11-45 

12.34 

13.22 

15.84 

2547 

75 

2.19 

3-07 

3-96 

4-84 

5-73 

6.62 

7-51 

8.40 

9.28 

10.17 

1  1.  06 

"•95 

12.84 

'5-33 

24.92 

So 

1.76 

2.65 

3-54 

4.42 

5-32 

6.21 

7.11 

8.00 

8.8S 

9.78 

IO.67 

11.57 

12.46 

14.82 

24-37 

35 

1.30 

2.22 

3-" 

4.00 

4.90 

5-8o 

6.70 

7-59 

8.48 

9-38 

10.28 

11.18 

12.07 

I4-32 

23.82 

90 

0.89 

I.78 

2.68 

3-58 

448 

5-38 

6.28 

7.18 

8.07 

8.98 

9-88 

10.78 

n.68 

13.81- 

23-27 

95 

0-45 

i-34 

2.25 

3-15 

4.05 

4.96 

5-86 

6.77 

7.66 

8.57 

9-47 

10.38 

11.29 

I3-3I 

22.73 

100 

o.oo 

0.90 

1.81 

2.71 

3.62 

4-53 

5-44 

6-35 

7.25 

8.16 

9.07 

9.98 

10.88 

12.80 

22.18 

the  greatest  economy  will  appear  where  the  temperature  of  the  flue  gases  was 
originally  the  highest  and  that  of  the  feed-water  the  lowest.  Even  with  a  low 
temperature  of  the  flue  gases,  an  economizer  will  usually  show  results  that  war- 
rant its  introduction. 


MECHANICAL   DRAFT. 


In  Table  No.  75  are  presented  the  results  of  a  number  of  tests  by  George 
H.  Barrus1  of  boiler  plants,  of  which  an  economizer  in  each  case  formed  a 
part.  In  the  first  four  cases,  designated  A,  B,  C  and  D,  the  temperature  of  the 
gases  as  they  leave  the  boiler  is  comparatively  low,  —  namely,  394°,  —  but  the 
initial  temperature  of  the  feed-water  is  raised  92°  on  the  average,  and  the  evap- 
oration increased  9.9  per  cent  per  pound  of  coal.  If  this  result  is  applied  to  a 
i,ooo-horse-power  plant,  and  the  cost  of  Cumberland  coal  for  a  day's  run  of 
10  hours  taken  as  $63.38,  per  Table  No.  58,  the  daily  saving  would  be  $6.27 
per  day.  For  a  year  of  308  working  days  this  represents  a  total  saving  of 
$1,931.16.  An  economizer  equipment  sufficient  to  secure  the  above  result 
would  probably  cost  from  $7,000  to  $8,000.  The  saving  would  represent  an 
annual  return  of  24  to  28  per  cent,  certainly  sufficient  to  warrant  careful  con- 
sideration. 

Table  No.  75.  —  Tests  with  Economizers. 


DESIGNATION  OF  BOILER. 

A 

n 

c 

D 

E 

Area  of  heating  surface,  boiler,  square  feet, 

1,894 

4,058 

5.592 

3,126 

1,  880 

Area  of  heating  surface,  economizer,  square  feet, 

1,  600 

1,920 

1,280 

i,  600 

1,  600 

Temperature  of  gases  leaving  boiler,  degrees, 

376 

361 

403 

435 

618 

Temperature  of  gases  leaving  economizer,  degrees, 

23I 

2  54 

299 

279 

365 

Temperature  of  feed-water  entering  economizer,  ) 
degrees,                                                               J 

95 

79 

III 

84 

88 

Temperature  of  feed-water  entering  boiler,  degrees, 

'75 

J45 

169 

196 

225 

Increased  evaporation  produced  by  economizer,  ) 

10.5           7.0 

9-3 

12.8 

29.0 

per  cent,                                                              ) 

Upon  the  same  basis  of  calculation,  the  case  designated  E  would  show  an 
annual  return  of  about  75  per  cent  of  the  investment.  Although  this  result  is 
only  approximate,  it  is  sufficiently  near  the  truth  to  indicate  the  indisputable 
economic  advantage  of  the  economizer.  The  cost  of  the  means  for  producing 
the  draft  requisite  to  such  results  will  be  found  to  be  less  in  the  case  of  a  fan 
than  of  a  chimney,  while  the  former  will  in  addition  possess  advantages  which 
make  it  much  the  more  desirable  of  the  two. 

At  the  steam  plant  of  the  Cheney  Brothers  silk  mills  at  South  Manchester, 
Conn.,  which  is  provided  with  an  economizer  and  Sturtevant  Mechanical  Draft 


Boiler  Tests.     George  H.  Barrus.     Boston, 


MECHANICAL   DRAFT. 


apparatus,  45  cubic  feet  of  water  is  heated  per  minute  and  used  in  the  boilers, 
while  an  additional  50  cubic  feet  is  heated  and  utilized  in  the  dyehouse.  The 
whole  quantity  is  raised  from  an  initial  temperature  of  112°  to  211°.  The  heat- 
ing of  the  feed-water  alone  is  sufficient  to  cause  a  saving  in  the  evaporative 
work  of  the  boiler  amounting  to  10  per  cent.  So  that  the  total  saving,  includ- 
ing the  heat  utilized  in  heating  water  for  the  dyehouse,  is  over  20  per  cent.  In 
a  plant  of  600  horse-power  running  10  hours  per  day,  and  using  coal  at  $4.00 
per  ton,  this  represents  an  annual  saving  of  about  $2,000  per  year,  an  excellent 
return  on  the  additional  investment  required  to  install  the  economizer  plant. 

The  influence  of  soot,  and  the  necessity  of  frequent  if  not  continuous  cleaning 
of  the  surfaces  of  an  economizer,  was  clearly  shown  by  M.  W.  Grosseteste1  in  a 
three  weeks'  test  with  smoky  coal  upon  a  Green  economizer,  consisting  of  a 
series  of  vertical  pipes  arranged  to  be  cleaned  externally  by  automatic  scrapers. 
The  apparatus  had  been  at  work  for  seven  weeks  continuously  without  having 
been  cleaned,  and  had  accumulated  a  half-inch  coating  of  soot  and  ash.  It  was 
observed  in  this  condition  throughout  the  first  week.  During  the  second  week 
it  was  cleaned  twice  every  day,  but  during  the  third  week,  after  having  been 
cleaned  on  Monday  morning,  it  was  worked  continuously  without  further  clean- 
ing. The  results  presented  in  Table  No.  76  show  the  necessity  of  cleaning. 

Table  No.  76.  —  Influence  of  the  State  of  the  Surface  of  an  Economizer. 


Temperature  of 
Feed-Water. 

Temperature  of 
Gaseuus  Products. 

H 

Hi 

»i 

TIME. 

ll 

S>J 

8 

HJ 

<*s 

8 

B9Q 

2* 

111 

"1 

•"'  3 

ll 

£ 

is 

11 

1 

r 

|sa 

1 

w  8 

>J  8 

5 

w  8 
a 

Jl 

5 

Pounds. 

Pounds. 

Pounds. 

First  week, 

73-5° 

161.5° 

88.0° 

849° 

261° 

588° 

2I4 

1,424 

6.65 

Second  week, 

77.0 

230.0 

i53-o 

882 

297 

585 

216 

1,525         7.06 

Third  week.     Monday, 

73-4 

196.0 

122.6 

831 

284 

547 

1 

Tuesday, 
Wednesday, 
Thursday, 

73-4 
79.0 
80.6 

181.4 
178.0 
170.6 

I08.0 
99-0 
90.O 

871 
952 

309 

562 
623 

L 

1,428 

6.70 

Friday, 

80.6 

169.0 

88.4 

889 

338 

551 

Saturday, 

79.0 

172.4 

93-4 

901 

35' 

55° 

j 

NOTE  TO  TABLE. —  The  averages  for  the  first  and  second  weeks  are  exclusive  of  Monday. 


Bulletin  de  la  Societe  Industrielle  de  Mulhouse,  Vol.  XXXIX.     1869. 


n6  MECHANICAL   DRAFT. 

Air  Heaters  or  Abstractors.  —  The  air  heater,  heat  abstractor,  warm-blast 
apparatus  or  hot-draft  apparatus,  as  it  is  variously  called,  generally  consists 
of  some  arrangement  of  pipes,  through  or  across  which  the  hot  gases  pass 
direct  from  the  uptake,  heating  thereby  the  air  supply  for  the  furnace,  which 
passes  respectively  across  or  through  the  pipes  ;  the  object  being  to  abstract 
from  the  gases  as  much  heat  as  is  practical  ^nd  transfer  it  to  the  air  before  it 
enters  the  furnace,  thereby  securing  a  higher  temperature  and  increased  evap- 
orative efficiency. 

Such  an  apparatus  is  virtually  the  equivalent  in  results  of  an  economizer, 
and  is  the  only  practical  means  of  reducing  the  waste  of  heat  in  the  flue  gases 
when  large  quantities  of  warm  water  are  not  in  demand,  as  must  be  the  case 
if  an  economizer  is  to  show  efficient  results.  The  ideal  arrangement,  however, 
consists  of  a  combination  of  economizer  and  abstractor,  whereby  the  air  sup- 
ply to  the  furnace  may  be  heated  as  well  as  the  feed-water  for  the  boiler,  and 
all  the  heat  practicable  thus  abstracted  from  the  flue  gases.  For  such  results 
chimney  draft  is  ordinarily  inadequate  and  mechanical  means  must  be  resorted 
to,  to  overcome  the  increased  resistance. 

Doubtless  the  most  comprehensive  test  ever  conducted  upon  an  apparatus 
of  this  character  was  that  undertaken  by  Mr.  J.  C.  Hoadley,1  in  the  interest 
of  a  number  of  mill  owners,  and  intended  to  determine  the  efficiency  of  the 
Marland  apparatus  for  heating  the  air  supply.  The  original  apparatus  was 
applied  to  a  single  horizontal  return  tubular  boiler,  60  inches  in  diameter,  with 
65  tubes  3^  inches  in  outside  diameter,  and  20  feet  long.  This  boiler  was  of 
the  regular  type  in  use  in  the  Pacific  Mills,  Lawrence,  Mass.,  where  the  tests 
were  made,  except  that  upon  its  top  were  placed  two  abstractors,  one  upon 
either  side,  3  feet  apart,  extending  the  entire  length  of  the  boiler.  Each 
abstractor  contained  120  lap-welded  tubes,  2  inches  in  outside  diameter  and 
20  feet  long,  enclosed  in  a  brick  setting.  Surrounding  each  pipe  was  a  3-inch 
tube  of  thin  iron.  By  proper  arrangement  of  the  heads  into  which  the  2-inch 
pipes  were  expanded,  in  connection  with  the  uptake,  a  passage  was  provided 
for  the  flue  gases  through  these  pipes  and  thence  to  the  blower  which  produced 
the  requisite  draft.  The  3-inch  tubes  were  shorter  than  the  2-inch,  and  were 
arranged  at  their  ends  so  that  air  for  the  furnace  could  pass  to  them  and  thence 
through  the  annular  space  between  the  two  tubes,  becoming  heated  by  the 
gases  in  the  inner  tube.  This  apparatus  was  known  as  Warm-Blast  No.  i. 
Alongside  this  boiler,  and  operating  under  the  same  conditions,  was  a  boiler 
of  the  regular  type,  designated  in  the  report  as  Pacific  Boiler. 


i  Warm-Blast  Steam-Boiler  Furnace.     J.  C.  Hoadley.     New  York,  1886. 


MECHANICAL   DRAFT. 


117 


Extended  experiment  having  shown  Warm  Blast  No.  i  to  be  incapable  of 
reducing  the  temperature  of  the  escaping  gases  below  160°,  the  Pacific  Boiler 
was,  accordingly,  converted  into  Warm-Blast  No.  2  by  placing  upon  its  top  two 
abstractors  differing  from  the  previous  ones,  and  constructed  substantially  as 
follows :  2-inch  spiral-locked  tubes  were  provided  for  the  passage  of  the  hot 
gases,  while  the  3-inch  tubes  were  replaced  by  a  series  of  deflectors  set  at  right 
angles  to  the  tubes,  which  passed  through  them.  These  deflectors  were  so 
arranged  that  air  entering  at  the  top  must  descend  across  and  among  the  2-inch 
tubes,  which  had  i-inch  spaces  between  them,  pass  under  the  first  deflector, 
then  rise  in  the  same  manner  and  pass  over  the  second  deflector,  and  so  on, 
until  the  air  passed  to  the  ashpit. 

The  comparative  temperatures  found  to  exist  in  connection  with  the  Pacific 
and  Warm-Blast  No.  i  boilers,  properly  reduced  for  comparison,  are  presented 
in  Table  No.  77.  These  figures  alone  seem  to  point  to  the  efficiency  of. the 

Table  No.  77. —  Comparative  Temperatures  in  Pacific  Boiler  and  Warm-Blast 
Boiler  No.  i. 


TEMPERATURES. 

Location  at  which  Temperature  was  Taken. 

Pacific  Boiler. 

Warm-Blast 
Boiler. 

Difference. 

In  heart  of  fire      .         

2,493° 

2,793° 

300 

At  bridge  wall       

1,340 

1,  600 

260 

At  pier           

895 

1,050 

'55 

In  smoke-box        

373 

375 

2 

Air  admitted  to  furnace         ..... 

32 

332 

300 

Steam  and  water  in  boiler     ..... 

300 

300 

0 

Gases  escaping  to  chimney  ..... 

373 

162 

211 

External  air  ........ 

32 

32 

0 

Gases  cooled,  Warm-Blast  Boiler 

213 

Air  warmed,  Warm-Blast  Boiler  .... 

300 

warm-blast  apparatus;  but  Table  No.  78,  comprising  the  important  economic 
results,  serves  to  indicate  more  definitely  the  relative  efficiency  of  the  various 
arrangements,  and  to  prove  the  marked  advantage  of  the  warm-blast  arrange- 
ment. Careful  tests  showed  that  the  power  consumed  in  driving  the  blower 
was  about  i  per  cent  of  the  whole  power  produced  by  the  boiler  in  combination 
with  a  good  steam  engine.  This  should  be  compared  with  the  much  larger 
expenditure  required  to  produce  the  draft  by  means  of  a  chimney. 


n8 


MECHANICAL   DRAFT. 


The  results  obtained  by  the  use  of  other  forms  of  air  heaters,  such  as  the 
Howden  and  the  Ellis  &  Eaves,  when  used  in  connection  with  a  fan  for  produc- 
ing draft,  will  be  discussed  in  a  succeeding  chapter. 

Table  No.  78. —  Results  of  Tests  with  Pacific  Boilers  and  Warm-Blast  Boilers. 


WITH  ANTHRACITE:  COAL. 

WITH  BITUMINOUS 
COAL. 

Pacific 
Boiler. 

Warm- 
mast 
No.  i. 

Warm- 
Blast 
No.  2. 

Pacific 
Boiler. 

Warm- 
Blast 

No.  i. 

Mean  temperature  of  external  air,  days,       degrees, 

78.30 

34° 

490 

710 

34.20 

Temperature  of  smoke-box,                            degrees. 

368.3 

396.9 

377 

376.9 

397-4 

Temperature  of  escaping  gases,                     degrees, 

368.3 

I89 

164 

376.9 

196 

Gases  cooled  by  abstractors,                          degrees, 

0 

207.9 

213 

O 

201.4 

Air  warmed  by  abstractors,                             degrees, 

0 

3°3-7 

285 

0 

3I5.5 

Temperature  of  air  supplied  to  furnace,       degrees, 

78.3 

337-7 

334 

71 

349-5 

Temperature  of  steam,                                    degrees, 

297.5 

361.1 

291.2 

297.3 

322.6 

Loss  at  chimney,                                              per  cent, 

17-75 

15.00 

12.83 

17-03 

14.24 

Loss  by  radiation  from  brickwork,               per  cent, 

2.64 

4.00 

4.00 

3-39 

4.00 

Loss  by  imperfect  combustion,                     per  cent, 

2.13 

0.63 

1-43 

2.85 

i.  06 

Total  loss  by  above  three  causes,                 per  cent, 

22.52 

19.63 

18.26 

23.27 

19.30 

Pounds  of  flue  gases  per  pound  of  coal, 

22.39 

23-49 

24.17 

25-23 

28.37 

Efficiency,  reduced  to  common  basis,          per  cent, 

68.87 

78.18 

81.43 

64.61 

77-59 

Difference  of  efficiency,  points  gained  by  Warm-  \ 
Blast  over  Pacific  Boiler,                                     J 

9-31 

12.56 

12.98 

Q.7I 

Ratio  of  gain  to  the  larger  quantity  (  g  Ig=II-9%) 

11.9 

15.4 

16.7 

Ratio  of  gain  to  the  smaller  quantity  (gg-gz  —  1  3-  5  %  ) 

13-5 

18.2 

20.1 

Increased  Tube  Heating  Efficiency.  —  With  fire  tubes  of  a  given  length  the 
amount  of  heat  transmitted  to  the  water  within  the  boiler  must  be  dependent 
upon  the  temperature  and  velocity  of  the  gases,  the  amount  of  surface  exposed, 
and  the  completeness  with  which  they  are  forced  into  contact  with  it.  In  other 
words,  with  the  same  velocity  and  temperature,  a  given  length  of  tube  will 
be  efficient  in  proportion  as  it  presents  absorbing  surface  for  receiving  the  heat 
of  the  gases,  and  as  those  gases  are  compelled  to  come  in  contact  with  it.  Two 
methods  are  in  use  for  accomplishing  this  result.  The  first  consists  in  fitting 
within  a  regular  boiler  tube  a  strip  of  thin  sheet-iron,  equal  in  width  to  the 
internal  diameter  of  the  tube,  and  twisted  so  as  to  form  a  helix  of  long  pitch, 
making  only  two  or  three  turns  in  the  length  of  the  tube.  The  effect  of  this 
arrangement  —  the  strip  being  known  as  a  "retarder"  —  is  to  break  up  the 


MECHANICAL   DRAFT. 


119 


current  of  gas  and  cause  all  portions  of  its  volume  to  touch  the  inner  surface  of 
the  tube.  At  the  same  time  the  retarder  itself  is  intensely,  heated,  and  rapidly 
radiates  its  heat  through  the  tube  to  the  water.  The  dual  effect  of  the  retarder 
is  to  materially  increase  the  evaporative  power  and  efficiency  of  the  boiler. 

The  economic  result  of  the  use  of  retarders  is  shown  by  the  tests  of  Mr.  J.  M. 
Whitham1  upon  a  i  oo-horse-power  horizontal  tubular  boiler  operated  at  from 
about  50  per  cent  below  to  about  140  per  cent  above  its  rated  capacity.  The 
evident  result  is  a  reduction  in  the  temperature  of  the  flue  gases,  with  a  corre- 
sponding decrease  in  the  coal  consumption.  This  is  shown  in  Table  No.  79. 

But,  evidently,  as  the  name  "  retarder  "  implies,  this  result  cannot  be  attained 
without  an  increase  in  the  draft.  In  this  connection  Mr.  Whitham  presents 
results,  given  in  Table  No.  80,  showing  the  different  drafts  and  resistances. 

Table  No.  79.  —  Reduction  in  Temperature  of  Flue  Gases  and  in  Coal  Consumption  by 
the  Use  of  Retarders. 


Horse-Power  Developed. 

Reduction  in  Temperature  of  Flue 
Gases. 

Degrees  Fahr. 

Reduction  in  Coal  Consumption. 
Per  cent. 

52 

20 

0.0 

75 

53 

0.0 

IOO 

32 

3-2 

I25 

46 

4.0 

150 

'9 

3-3 

170 

59 

3-6 

200 

36 

4.1 

225 

26 

8.6 

239 

123 

18.4 

Naturally,  this  additional  draft  can  be  most  readily  obtained  when  mechanical 
means  are  employed.  In  fact,  retarders  have  been  most  extensively  introduced 
where  mechanical  draft  is  in  use. 

Mr.  Whitham's  general  deductions  regarding  retarders  are  that  they  inter- 
pose a  resistance  varying  with  the  rate  of  combustion  ;  that  they  reduce  the 
temperature  of  the  flue  gases,  and  increase  the  effectiveness  of  the  heating  sur- 
face ;  that  they  should  not  be  used  where  the  draft  is  small ;  that  they  can  be 
used  to  advantage  in  plants  using  a  fan,  and  that  they  may  show  from  5  to  10 
per  cent  advantage  whenever  the  boiler  plant  is  pushed  and  the  draft  is  strong. 


i  The  Effect  of  Retarders  in  Fire  Tubes  of  Steam  Boilers.     J.  M.  Whitham.     Transactions 
American  Society  of  Mechanical  Engineers,  Vol.  XVII. 


MECHANICAL   DRAFT. 


Table  No.  80.  —  Draft  and  Resistance  when  Retarders  are  Used. 


Draft  or  Resistance  in  Inches 
of  Water. 


Furnace  draft 

Resistance  of  pass  under  boilers  and  through  tubes  without  | 
retarders,  ) 

Total  draft  of  stack  if  no  top  is  used       .... 

Resistance  due  to  having  retarders 

Total  draft  if  there  is  no  return  pass  and  retarders  are  used, 
Increased  resistance  due  to  return  pass  over  top  of  boilers, 


0.30 
0.27 

o-57 
0.31 
0.88 
0.07 


The  second  method  of  increasing  the  efficiency  of  fire  tubes  is  more  direct, 
and  consists  in  a  special  construction  of  the  tube  itself.  Such  is  the  case  in 
the  Serve  tube,  which  is  outwardly  cylindrical,  but  from  its  inner  or  fire  surface 
a  number  of  equidistant  radial  ribs  parallel  to  the  axis  converge  toward  the 
centre.  The  radial  length  of  the  ribs  is  usually  about  one-fifth  of  the  external 
diameter  of  the  tube,  and  they  are  seven  or  eight  in  number,  according  to  the 
external  diameter.  The  superior  economy  of  these  tubes  is  accounted  for  upon 
the  theory  that  the  ribs  break  up  the  column  of  gases,  and  by  means  of  their 
extended  surface  extract  heat  from  all  parts  of  it.  A  six-day  comparative  test 
of  the  efficiency  of  plain  and  ribbed  tubes,  under  practically  identical  condi- 
tions, was  made  by  Mr.  H.  B.  Roelker,1  and  the  general  results  are  presented 
in  Table  No.  81.  In  some  cases  retarders  are  used  with  these  tubes,  securing 
thereby  even  better  contact  of  the  air,  because  of  its  being  forced  to  pass 
through  the  spaces  between  the  retarder  and  the  ribs. 

Table  No.  81.  —  Comparative  Tests  of  Efficiency  of  Ribbed  and  Plain  Fire  Tubes. 


«H              & 

Pounds  of  Water  Evao- 

|d| 

orated  per  P 

aund  of  Coal 

Manner 

DURATION  OF  TESTS. 

of  Producing 

T».~,** 

111 

Draft. 

£  | 

Plain 

Ribbed 

Tubes. 

Tubes. 

Two  8-hour  tests          ...... 

Natural 

ys       5.08 

7-35 

Two  8-hour  tests          .         .         .         ,     ;._        . 

Mechanical        ^            5.98 

7.6o 

Two  8-hour  tests          .         .         .        ... 

Mechanical 

H           4-68 

6-75 

One  8-hour  test    .         .         . 

Mechanical 

i  3-16 

7.41 

One  8-hour  test    ...         .         .  -      .         . 

Mechanical 

i  9-16 

6.52 

i  Serve's  Ribbed  Boiler  Tube.     Passed  Assistant  Engineer  G.  S.  Willits,  U.  S.  Navy.     Jour- 
nal of  American  Society  of  Naval  Engineers,  August,  1891. 


MECHANICAL   DRAFT.  121 

Of  Course  the  highest  relative  efficiency  of  such  devices  will  be  shown  when 
the  tubes  are  comparatively  short  and  the  gases  with  plain  tubes  are  rejected 
at  a  high  temperature.  But  under  all  ordinary  conditions  there  can  be  no 
question  of  their  efficiency.  They  are  of  especial  advantage  when  space  or 
the  design  of  the  boiler  forbids  the  convenient  introduction  of  additional  heat' 
ing  surface  in  any  other  manner,  as  is  usually  the  case  in  marine  boiler 
practice.  They  cannot,  however,  be  advantageously  used  when  the  draft  is 
small,  but  are  sources  of  decided  economy  in  mechanical  draft  plants.  In  fact, 
under  most  conditions  mechanical  draft  is  necessary  to  their  success,  and 
with  it  they  are  quite  extensively  employed. 

Mechanical  Stokers.  —  The  higher  efficiency  attained  when  the  firing  is  in 
small  amounts  at  frequent  intervals,  and  when  the  fire  is  carefully  maintained 
in  the  best  possible  conditions,  points  to  the  results  which  should  ensue  from 
the  employment  of  a  proper  method  of  continuously  feeding  the  fire  by 
mechanical  means.  The  mechanical  stoker,  as  a  substitute  for  hand-firing,  pos- 
sesses many  advantages,  but  they  can  only  be  realized  when  the  stoker  is  prop- 
erly suited  to  its  particular  work  and  is  intelligently  operated.  Its  advantages 
may  be  summarized  as,  — 

First.  Adaptability  to  the  economical  combustion  of  the  cheapest  grades 
of  fuel. 

Second.    Saving  in  labor  of  firing. 

Third.  Economy  in  combustion  even  with  forced  firing,  under  proper  man- 
agement. 

Fourth.    Constancy  and  uniformity  of  the  furnace  conditions. 

Fifth.    Smokelessness. 

Three  principal  types  of  mechanical  stokers  are  to  be  found  in  use. 

The  under  feed,  by  which  the  fuel  is,  by  screw  or  plunger,  forced  upward 
from  beneath.  In  the  common  forms  the  fuel  is  thus  supplied  along  the  centre 
of  the  length  of  the  grate,  and  as  it  is  forced  upward  falls  over  to  the  sides  and 
thus  forms  a  long  mound,  thin  at  the  sides  of  the  grate  and  of  considerable 
thickness  in  the  centre.  This  thickness  necessitates  a  very  strong  under-grate 
blast,  which  can  only  be  secured  by  the  use  of  a  blower,  and  is  generally 
applied  in  greatest  volume  at  or  near  the  centre. 

The  inclined  over-feed  type  generally  consists  of  a  sloping  grate,  the  highest 
portion  being  at  the  front  of  the  boiler,  where  the  coal  is  fed.  The  grate  bars 
are  usually  so  constructed  and  arranged  that  they  may  be  periodically  moved 
so  as  to  feed  the  fuel  along  and  down  the  surface.  The  motive  power  by  which 
this  practically  continuous  movement  and  feeding  process  is  maintained  is, 
in  most  cases,  derived  from  a  small  independent  engine,  although  hand  power 


MECHANICAL   DRAFT. 


is  sometimes  used  in  small  plants.  Evidently  this  latter  method  of  operating 
results  in  a  much  less  frequent  movement  of  the  coal.  The  best  results  with 
this  form  of  mechanical  stoker  are  usually  obtained  when  forced  draft  is  used 
and  the  air  is  admitted  either  through  the  grate  bars  themselves,  or  between 
them  from  the  space  beneath.  With  either  arrangement  there  is  an  excellent 
opportunity  for  the  most  perfect  distribution  of  the  air  and  its  intimate  contact 
with  the  fuel. 

The  third  type  of  mechanical  stoker  consists  of  a  chain  grate  upon  which 
the  fuel  is  fed  at  the  front  of  the  boiler,  and  which  by  its  slow  progress  toward 
the  bridge  wall  gives  the  fuel  an  excellent  and  undisturbed  opportunity  for  com- 
plete combustion.  It  is  particularly  adapted  for  the  lowest  grades  of  fuel,  and 
in  its  most  perfect  form  is  so  arranged  that  the  fuel,  at  different  points  in  its 
progress,  receives  its  air  supply  in  different  amounts  and  under  different  pres- 
sures, each  best  suited  to  the  given  stage  of  the  combustion.  The  proper 
introduction  and  regulation  of  the  air  requires  that  it  should  be  supplied  by  a 
blower,  which  thus  forms  an  inherent  part  of  such  a  plant.  Comparisons 
between  mechanical  stoking  and  hand-firing,  as  well  as  between  different  forms 
of  mechanical  stokers,  demand  that  the  conditions  shall  be  practically  identical. 
In  Table  No.  82  are  presented  the  results  of  several  comparative  tests  made 
under  such  conditions,  each  with  a  different  form  of  mechanical  stoker.  They 

Table  No.  82.  — Tests  of  Mechanical  Stokers. 


A 

B 

C 

D 

Hand. 

Stoker. 

Hand. 

Stoker. 

Hand. 

Stoker. 

Hand. 

Stoker. 

Coal  per  hour,  pounds, 

2,O22 

1,422 

2,022 

1,  800 

428 

432 

857 

771 

Coal  per  hour  per  sq.  ft.  grate,  pounds, 

2  5-3 

28.4 

25'3 

29.0 

* 

29.1 

3°-9 

Water  evaporated  per  hour  per  pound  ) 
of  coal  from  and  at  2  1  2°,  pounds,     \ 

7.11 

8.70 

7.II 

9.12 

7.68 

8.80 

8.8  1 

10.29 

Water  evaporated  per  hour  per  pound  } 
of   combustible  from  and  at  2120,  \ 

7-77 

9.67 

7-77 

10.07 

8.96 

10.01 

pounds,                                                    ) 

serve  to  show  the  undeniable  economy  resulting  from  this  method  of  feeding 
coal.  Although  several  forms  of  mechanical  stokers  are  included  in  the  list, 
the  tests  are,  for  obvious  reasons,  designated  only  by  distinguishing  letters, 
each  letter  covering  the  results  with  both  hand-firing  and  mechanical  stoking 
under  similar  conditions. 


MECHANICAL   DRAFT.  123 

Constant  attention  is  necessary  in  mechanical  firing  in  order  to  regulate  the 
rate  of  feed  to  the  rate  of  evaporation ;  but  the  total  amount  of  labor  is  far  less 
than  that  required  in  hand-firing.  When  bituminous  coal  is  mechanically  fired 
there  can  be  but  little  question  that  the  plant,  if  of  reasonable  size,  will  operate 
with  sufficient  economy  to  pay  a  good  return  on  the  extra  investment  required 
for  the  stoking  apparatus. 

Powdered  Fuel  Furnaces.  —  Coal,  in  the  form  of  dust,  fed  to  the  boiler  furnace 
in  a  current  of  air,  has  to  some  extent  been  employed  for  the  purposes  of  steam 
generation.  The  arrangement  usually  comprises  a  device  for  reducing  the  coal 
to  an  impalpable  powder.  It  is  then  fed,  together  with  air  ordinarily  supplied 
by  a  fan,  into  the  front  of  the  furnace.  Theoretically,  this  method  appears  to 
have  certain  advantages  which  should  make  it  successful.  There  is  opportunity 
for  instantaneous  combustion,  and  the  most  intimate  contact  of  the  air,  whereby 
the  minimum  amount  may  be  employed.  There  should  be  no  loss  by  decrepita- 
tion ;  but  this  is  more  than  offset  by  the  tendency  to  blow  the  dust  in  an  uncon- 
sumed  state  directly  up  the  chimney.  The  results  indicate,  however,  that  with 
such  methods  as  have  been  tested  the. gain,  if  any,  is  more  than  counter- 
balanced by  the  added  expense. 

Influence  of  Mechanical  Draft  on  the  Ultimate  Efficiency  of  Steam  Boilers. — 
Several,  although  by  no  means  all,  of  the  advantages  of  mechanical  draft  as  a 
means  of  increasing  boiler  efficiency  and  capacity  have  already  been  pointed 
out.  By  its  use  the  draft  is  rendered  positive,  and  the  air  required  for  a  given 
weight  of  fuel  may  be  reduced  to  a  minimum.  As  stated  by  Mr.  Richard  Sen- 
nett/  it  "tends  to  promote  economy  of  fuel  in  consequence  of  the  better  supply 
of  air  and  the  higher  temperature  at  which  the  fires  are  worked." 

The  efficiency  of  a  boiler  plant  from  a  commercial  standpoint  —  and  that  is 
the  point  from  which  it  must  ultimately  be  judged  —  concerns  not  alone  the 
cost  of  the  fuel,  and  the  cost  of  handling  and  firing  the  same,  but  also  the  cost 
of  the  boiler  plant  itself,  including  the  space  it  occupies  in  stationary  or  marine 
practice,  with  all  of  the  appurtenances  designed  to  facilitate  the  economical 
production  of  steam,  and  the  means  of  housing  or  protecting  the  same.  Upon 
this  broad  basis  should  be  judged  the  efficiency  of  mechanical  draft  as  a  most 
important  factor  in  modern  steam-boiler  practice.  Its  many  advantages,  as 
clearly  proven  by  extended  use  and  careful  trial,  will  be  presented  at  length  in 
succeeding  chapters. 

It  is  proper  here,  however,  to  consider  the  influence,  from  a  commercial  stand- 
point, which  the  application  of  mechanical  draft  exerts  upon  the  ultimate  effi- 

i  Closed  Stokeholds.  Richard  Sennett.  Transactions  of  Institution  of  Naval  Architects, 
1886. 


124 


MECHANICAL   DRAFT. 


ciency  of  a  steam-boiler  plant  as  measured  by  its  aggregate  first  cost  and  the 
consequent  fixed  charges  thereon.  For  this  purpose  there  has  been  selected  a 
plant  of  reasonable  size  of  which  the  detailed  cost  is  known.  This  plant,  as 
illustrated  in  plan  in  Fig.  i,  ^^f^^^  consists  of  8  modern  water-tube 
boilers,  each  of  200  horse-power  ^^^^%\  nominal  rating,  set  in  pairs, 
making  a  total  of  1,600  horse-  I  §>™NE^  •  power.  A  chimney  is  provided 
8  feet  in  internal  diameter  by  ^fl"~Hr  I^°  *eet  n^§n'  °^  surncient  ca- 
pacity to  overcome  the  resist-  T — T  ance  of  the  economizers 


BOILERS 


BOILERS 


BOILERS 


FIG.  i. 

and  produce  the  draft  necessary  for  any  probable  forcing  of  the  boilers.  Two 
feed-water  economizers  are  provided,  through  or  around  which  the  gases  may 
be  caused  to  pass  on  their  way  to  the  chimney.  The  boilers  and  economizers 
are  enclosed  in  an  independent  boiler  house,  outside  which  stands  the  chimney. 
The  detailed  cost  of  that  portion  of  the  plant  which  concerns  the  present  dis- 
cussion is  in  round  numbers  as  follows :  — 


8  water-tube  boilers  of  200  horse-power  each, 
2  feed-water  economizers         .... 
Boiler  and  economizer  setting  and  by-pass 
Automatic  damper  regulator  and  dampers 
Chimney,  complete         .         .  '  . 

Building,  complete          ...         .         .         y 


$25,000.00 

7,000.00 

6,000.00 

300.00 

9,000.00 

1 1,000.00 

$58,300.00 


Being  taken  in  round  numbers,  these  costs  may  be  considered  to  represent  a 
fair  average  for  the  vicinity  of  New  England.  The  cost  of  the  chimney  includes 
the  foundations  necessary  where  the  ground  is  stable,  but  there  is  always  a  pos- 


MECHANICAL    DRAFT. 


I25 


sibility  of  considerable  increased  cost  when  the  nature  of  the  ground  demands 
deeper  or  more  extended  foundations. 

In  Figs.  2  and  3  is  shown  the  simplified  arrangement  which  is  possible  when 


ney.     This  ap- 
to  produce  the 


a  mechanical  draft  apparatus  is  substituted  for  the  chim- 
paratus,  of  the  induced  type,  is  of  sufficient  capacity 
same  maximum  draft  as  the  chimney,  and 
may  be  readily  placed  on  top  of  the  econ- 
omizers without  occupying  space  other- 
wise valuable.  The  apparatus  as  shown 
in  the  illustrations  is  of  the  duplex  type, 
consisting  of  two  steel-plate  fans  placed 
side  by  side,  each  driven  by  a  double- 
cylindered  upright  engine.  Each  fan  as 
designed  is  capable  of  independently  pro- 
ducing the  draft  for  the  entire  plant,  and 
may,  therefore,  be  operated  alone  at  near  its  maximum  speed,  or  both  fans  may. 
be  driven  at  less  speed  to  accomplish  the  same  results.  This  duplex  arrange- 
ment with  fans  in  duplicate  is  not  positively  necessary  except  in  cases  where 


FIG.  2. 


BOILERS 


BOILERS 


BOILERS 


BOILERS 


FIG.  3. 

the  work  required  is  practically  continuous,  and  where  any  delay  from  possible 
accident  to  the  apparatus  would  cause  great  inconvenience.  By  means  of  an 
arrangement  of  dampers  in  the  inlet  connection  between  the  fans,  the  hot  gases 
drawn  from  the  space  beneath  may  be  caused  to  pass  through  either  of  the  two 
fans,  while  access  may  be  had  to  the  interior  of  the  other  without  inconvenience 
from  the  heat.  By  means  of  a  special  form  of  draft  regulator  the  fans  are  kept 
continuously  in  operation  at  just  the  speed  necessary  to  produce  the  draft  that, 
with  different  conditions  of  the  fire,  will  maintain  the  steam  pressure  constant. 


126  MECHANICAL   DRAFT. 

This  is  one  of  the  most  important  features  of  such  an  arrangement,  for,  as  the 
conditions  of  the  fire  change,  it  is  obvious  that  the  intensity  of  the  draft  and 
the  amount  of  air  supplied  should  change  in  like  proportion.  In  a  subsequent 
chapter  are  shown  coincident  record  cards  from  draft  and  steam-pressure  record- 
ing gauges,  which  display  the  conditions  which  are  maintained  by  mechanical 
draft.  The  gases  leaving  the  fans  pass  to  a  short  connecting  steel  stack  extend- 
ing through  the  roof. 

The  same  general  arrangement  of  economizer,  by-pass  and  dampers  may  be 
preserved  ;  while  the  chimney  and  the  space  occupied  thereby  are  rendered 
unnecessary.  Such  an  apparatus,  with  its  duplicate  engines  and  fans,  its 
automatic  regulation  and  the  short  stack,  can  be  installed  complete,  under 
ordinary  conditions,  for  about  $3,500.  Evidently,  the  cost  would  be  much  less 
were  the  duplicate  feature  omitted,  and  only  a  single  fan  of  the  requisite  capac- 
ity employed.  As  the  special  draft  regulator  takes  the  place  of  the  damper 
regulator  usually  employed  in  connection  with  the  chimney,  the  total  economy 
in  first  cost  effected  by  the  introduction  of  the  mechanical  draft  plant  may  be 
indicated  thus  ;  the  saving  of  space  occupied  by  .the  chimney  being  neglected  :  — 

Chimney  Draft.  Mechanical  Draft. 

Cost  of  chimney       .         .        $9,000.00   I   Cost  of  fans,  engines,  draft 
Cost   of  damper  regulator  regulator,  and  short  stack, 

and  dampers         .          .  300.00          installed  complete         .       $3,500.00 

Saving  by  use  of  mechanical 

draft    ....         5,800.00 

$9,300.00   ;  $9,300.00 

It  is  thus  evident  that  the  mechanical  draft  apparatus  here  costs  only  about 
38  per  cent  of  the  chimney  and  damper  regulators.  Had  only  a  single  fan  of 
the  requisite  capacity  been  used  instead  of  the  relay  arrangement  shown,  the 
cost  of  the  mechanical  draft  apparatus  would  have  been  reduced  to  about 
$2,000,  making  its  relative  cost  only  about  22  per  cent  of  the  chimney  and  draft 
regulator,  and  the  saving  would  amount  to  $7,300. 

A  still  further  reduction  might  have  been  secured  by  designing  the  plant  so 
as  to  operate  the  boilers  at  somewhat  above  their  rated  capacity,  as  could  be 
readily  done  by  means  of  a  mechanical  draft  apparatus,  and  in  fact  by  means  of 
the  same  apparatus  upon  which  the  cost  has  already  been  given.  Although 
such  reduction  would  most  naturally  be  made  in  the  rated  capacity  of  each 
boiler,  yet,  for  illustration,  it  would  be  simpler  to  consider  the  effect  of  omit- 
ting one  of  the  boilers  of  the  plant  as  designed.  This  would  bring  the  rated 


MECHANICAL   DRAFT. 


127 


capacity  down  to  1,400  horse-power,  and  would  call  upon  the  fans  to  only 
increase  the  steaming  capacity  of  the  other  boilers  by  about  14  per  cent  above 
the  normal.  This  would  show  an  additional  saving  in  first  cost  which  may  be 
thus  presented :  — 


1, 600  Nominal  Horse-Power  Plant. 

Cost  of  8  boilers  .  .  $25,000.00 
Cost  of  settings,  etc.  .  6,000.00 
Cost  of  building  .  .  11,000.00 


$42,000.00 


1,400  Nominal  Horse-Pou>er  Plant. 

Cost  of  7  boilers,  .  .  $21,875.00 
Cost  of  settings,  etc.,  about  5,500.00 
Cost  of  building,  about  .  10,500.00 
Saving  by  use  of  mechanical 

draft    ....         4,125.00 

$42,000.00 


This  shows  a  possible  supplementary  saving  on  the  entire  plant  of  $4,125, 
which,  in  addition  to  the  direct  saving  already  shown  to  be  effected  by  the  intro- 
duction of  mechanical  draft,  making  no  allowance  for  reduced  expense  on 
account  of  less  space  occupied,  makes  a  total  reduction  of  $9,925  to  be  credited 
to  the  account  of  the  mechanical  method.  This  saving  is  equal  to  more  than 
the  entire  cost  of  chimney  and  damper  regulator,  and  to  nearly  three  times  the 
first  cost  of  the  mechanical  draft  plant.  Of  course,  the  fixed  charges  for 
interest,  taxes  and  insurance  will  be  correspondingly  reduced.  Had  this  com- 
parison been  based  upon  the  cost  of  a  forced  draft  plant, —  that  is,  one  in  which 
the  air  was  forced  rather  than  drawn  through  the  fire, —  the  saving  in  the  cost 
would  have  been  shown  to  be  even  greater ;  for  a  single  fan  would  probably 
have  served  the  purpose,  and,  being  employed  to  handle  the  air  at  atmospheric 
pressure,  would  not  require  to  be  as  large  as  the  induced  draft  plant,  through 
which  must  pass  the  air  and  gases  when  nearly  doubled  in  volume  by  their 
increase  in  temperature. 

In  any  properly  arranged  power  plant  there  should  be  no  question  whatever 
as  to  the  opportunity  to  utilize  the  exhaust  steam  from  the  fan  engine.  This 
may,  and  in  a  plant  of  proper  design  would,  be  used  for  heating  buildings,  for 
feed-water  heating,  or  would  be  eventually  saved  by  condensation  and  return  to 
the  boilers.  Under  such  conditions  the  actual  cost  of  the  steam  used  in  produc- 
ing draft  would  be  reduced  to  practically  nothing. 

In  this  estimate  the  cost  of  the  land  occupied  by  the  chimney  or  the  invested 
capital  sufficient  to  earn  the  rental  rate  thereon  has  been  neglected ;  but  if  this 
were  an  electric  lighting  or  power  plant  situated  in  the  heart  of  a  large  city, 
this  would  also  form  an  important  factor  in  the  economy  of  the  mechanical 
draft  system.  The  space  occupied  by  the  chimney,  measuring  the  latter  by  the 


128  MECHANICAL   DRAFT. 

area  of  the  enclosing  rectangle,  amounts  to  495  square  feet,  which,  at  a  nominal 
rate  of,  say,  $2  per  square  foot,  would  cost  $990.  A  similar  allowance  made 
for  the  decreased  space  resulting  from  a  reduction  in  the  number  of  boilers 
while  maintaining  the  same  steaming  capacity,  would  show  a  saving  at  $2 
per  foot  upon  480  square  feet,  or  a  total  of  $960.  Of  course  these  items  de- 
pend entirely  upon  the  actual  cost  of  the  land,  and  will  vary  greatly  according 
to  its  location. 

The  total  net  saving  in  first  cost  of  a  single  plant,  under  the  given  conditions, 
may  be  thus  summarized  :  — 

By  omission  of  chimney  and  damper  regulator  .  $5,800.00 

By  reduction  in  number  of  boilers      .          .  .  4,125.00 

By  saving  in  space  occupied  by  chimney    .  .  990.00 

By  saving  in  space  occupied  by  boiler  omitted  .  960.00 

$i  1,875.00 

This  saving  is  made  possible  by  the  expenditure  of  $3,500  for  the  mechanical 
draft  apparatus  ;  that  is,  the  saving  is  nearly  three  and  a  half  times  the  expendi- 
ture necessary  to  secure  it.  Had  it  been  desired  to  secure  the  desired  1,600 
horse-power  by  increasing  the  steaming  capacity  of  the  boilers  above  the  mere  14 
per  cent  here  estimated  upon,  the  saving  would  have  been  greater. 

The  reduction  of  $11,875  ^n  the  cost  w°uld  indicate  an  annual  saving  in 
fixed  charges  of  about  $831  to  $890,  according  as  the  aggregate  of  interest, 
taxes  and  insurance  is  taken  at  7  or  -j*4  per  cent.  This  amount  would,  under 
conditions  of  the  best  economy,  be  practically  sufficient  to  cover  the  cost  of 
operating  the  mechanical  draft  apparatus,  provided  no  attempt  was  made  to 
utilize  the  exhaust  steam.  When  to  this  is  added  the  increased  convenience  of 
mechanical  draft,  its  positive  character,  its  ready  adaptability,  its  independence 
of  climatic  conditions,  and  its  instant  response  to  any  demand  for  increased 
steam  supply,  the  account  stands  decidedly  to  its  credit.  Therefore  any  further 
saving,  as  for  instance  in  the  cost  of  the  fuel  burned,  is  clear  gain  over  and 
above  any  expenditure  which  may  have  been  made  on  account  of  the  introduc- 
tion of  this  method. 

In  another  light,  however,  the  saving  in  fuel  cost  may  be  considered  as  regards 
the  amount  per  ton  which  the  price  would  have  to  be  reduced  in  order  to  cover 
the  cost  of  operating  the  fan  in  case  all  of  the  exhaust  steani  is  thrown  away. 
We  may  take  for  illustration  the  same  1,600  horse-power  plant.  Upon  the  com- 
mercial horse-power  rating  of  34.5  pounds  of  water  evaporated  from  and  at 
212°,  this  would  indicate  a  total  hourly  evaporation  of  55,200  pounds  of  water. 
If  1 1  pounds  of  water  were  evaporated  per  pound  of  coal  under  these  condi- 


MECHANICAL   DRAFT,  129 

tions,  this  would  call  for  55>200  =  5,018  pounds  of  coal  per  hour,  or  2.509  tons 

of  2,000  pounds  each.  Estimating  on  312  working  days  of  10  hours  each,  this 
requires  7,828  tons  per  year.  A  saving  of  only  15  cents  per  ton  effected  by  the 
installation  of  a  mechanical  draft  plant  would '  indicate  a  reduction  in  the  fuel 
account  of  $1,174.20  per  year,  an  amount  sufficient  under  ordinary  conditions  to 
pay  the  full  cost  of  operating  the  fan,  provided  it  was  impossible  to  utilize  the 
exhaust  steam.  Whatever  net  amount  can  be  saved  per  ton,  over  and  above 
this  amount,  can  be  credited  to  the  mechanical  draft  and  added  to  the  reduction 
in  fixed  charges.  When  it  is  known  that  a  saving  in  cost  of  coal  of  from  $50 
to  $75  per  week  may  be  readily  shown  in  a  plant  of  this  size  and  character, 
the  economical  possibilities  in  the  way  of  mechanical  draft  are  evident  beyond 
question.  Such  a  weekly  saving  is  equivalent  to  an  annual  reduction  in  the 
fuel  cost  of  from  $2,600  to  $3,900,  certainly  a  most  excellent  return  on  an 
investment  of  only  $3,500,  or,  rather,  on  an  original  net  saving  of  $11,871;.  In 
the  preceding  chapter  has  been  instanced  the  case  of  a  boiler  plant  of  1,005 
horse-power  nominal  rating,  in  which  the  fuel  saving  due  to  the  introduction  of 
mechanical  draft  amounted  to  $126  per  week.  This  is  equivalent  to  an  annual 
saving  of  $6,552,  or  far  more  than  the  original  cost  of  the  draft  plant. 

Although  by  no  means  all  of  the  factors  concerned  in  the  increased  ultimate 
efficiency  of  steam  boilers  have  been  here  considered,  nevertheless  those  pre- 
sented have  been  sufficient  to  clearly  point  to  the  economical  advantages  of 
mechanical  draft  as  relating  to  its  first  cost.  Additional  influences  upon  the 
economy  effected  by  this  method  of  draft  production  will  be  presented  in  suc- 
ceeding chapters. 


CHAPTER   VII. 
RATE   OF   COMBUSTION. 

Rate  of  Combustion.  —  The  rate  at  which  fuel  is  consumed  in  steam-boiler 
practice  is  usually  expressed  in  pounds  per  hour  per  square  foot  of  grate  sur- 
face. Evidently,  this  rate  must  vary  greatly  according  to  the  conditions  govern- 
ing in  a  given  case.  Although  the  draft  is  by  far  the  most  important  factor,  yet 
the  amount  of  coal  burned  per  unit  of  area  also  depends,  to  a  certain  extent, 
upon  the  total  area  and  arrangement  of  the  grate  surface,  the  method  of  firing, 
the  kind  of  fuel,  and  other  factors  of  less  importance. 

In  the  early  days  of  the  steam  boiler  the  rate  of  combustion,  like  the  speed 
of  the  first  steam  engines,  was  extremely  low.  The  grates  were  large,  the  draft 
comparatively  light,  and  forcing  of  the  boilers  less  common  than  at  the  present 
day.  The  general  rates  of  combustion,  as  given  by  Rankine1  for  various  types 
of  boilers  and  conditions  of  draft,  are  presented  in  Table  No.  83. 

Table  No.  83.  —  Rates  of  Combustion. 


CONDITIONS. 

Pounds  of  Coal 

Method  of 

per  Square  Foot 

Producing  Draft 

of  Grate  per 

Hour. 

Slowest  rate  of  combustion  in  Cornish 

boilers 

Chimney. 

4 

Ordinary  rates  in  Cornish  boilers 

Chimney. 

10 

Ordinary  rates  in  factory  boilers 



Chimney. 

12  tO   I  6 

Ordinary  rates  in  marine  boilers 

Chimney. 

1  6  to  24 

Quickest  rates  of  complete  combustion 
air  coming  through  the  grate  only, 

of  dry  coal,  the  supply  of  ^ 

Chimney. 

20  to  23 

Quickest  rates  of  complete  combustion  of  caking  coal,  with  air  ) 
holes  above  the  fuel  to  the  extent  of  1-36  area  of  grate,            \ 

Chimney. 

24  to  27 

Locomotives        ..... 

! 

Blast  pipe 
or  fan. 

>    40  to  1  20 

Since  these  figures  were  first  given  the  rates  have  increased,  so  that  15  to  20 
pounds  is  not  at  all  uncommon  in  factory  boilers,  under  exceptionally  strong 
draft;  while  25  to  40  pounds  is  the  usual  marine  practice,  and  60  to  125  pounds 


The  Steam  Engine  and  Other  Prime  Movers.     W.  J.  M.  Rankine.     London,  1885. 


MECHANICAL   DRAFT. 


are  burned  in  the  boilers  of  torpedo  boats.  In  fact,  modern  steam-engineering 
practice  is  constantly  looking  toward  higher  rates  of  combustion  as  an  accom- 
paniment to  higher  steam  pressures  and  engine  speeds,  in  the  attempt  to  attain 
increased  efficiency. 

Mr.  W.  S.  Hutton1  states  that  "the  application  of  forced  draft  to  a  furnace 
affords  a  means  of  obtaining  a  higher  rate  of  combustion  of  fuel  per  square  foot 
of  grate  surface  per  hour  than  is  conveniently  obtainable  with  natural  draft. 
The  rate  of  combustion  obtained  in  practice  varies,  with  the  intensity  of  the 
draft,  from  30  to  200  pounds  of  coal  per  square  foot  of  fire-grate  surface  per 
hour.  A  moderate  rate  of  forced  combustion  is  from  35  to  50  pounds  of  coal 
per  square  foot  of  grate  per  hour." 

Relation  of  Grate  Surface  to  Heating  Surface.  —  The  ratio  existing  between 
the  areas  of  grate  and  heating  surface  in  various  types  of  boilers  has  alreadv 
been  presented  in  Table  No.  72.  The  influence  of  this  ratio  on  the  evaporation 
has  also  been  indicated.  As  the  economy  of  the  boiler  is  usually  expressed  in 
the  number  of  pounds  of  water  evaporated  per  pound  of  coal,  it  is  evident  that, 
the  surface  ratio  and  the  rate  of  combustion  also  being  known,  the  relative 
capacity  of  the  boiler  can  be  determined  and  expressed  in  pounds  of  water 
evaporated  per  square  foot  of  heating  surface.  In  fact,  this  should  be  the  ulti- 
mate basis  of  comparison  rather  than  the  rate  of  combustion ;  for  the  influence 
of  the  latter  is  dependent  upon  the  surface  ratio,  which  may  vary  considerably 
even  in  boilers  of  the  same  type.  The  results  of  this  influence  are  shown  in 
Table  No.  84,  in  which,  with  a  constant  fuel  efficiency  of  10  pounds  of  water 

Table  No.  84.  —  Rates  of  Evaporation  per  Square  Foot  of  Heating  Surface. 


•  • 

POUNDS  OF  COAL  PER  HOUR  PER  SQUARE  FOOT  OF  GRATB. 

SURFACE 

RATIO. 

; 

5 

.0 

'5 

2O 

25             30 

35 

40 

45 

:25 

2.00 

4-00 

6.00 

8.00 

10.00 

I2.0O 

14.00 

16.00 

1  8.  oo 

:3o 

1.67 

3-33 

5.00 

6.66 

8.33 

10.00 

11.67 

J3-33 

15.00 

=  35 

1-43 

2.86 

4.29 

5-72 

7-15 

8.58 

IO.OO 

11.44 

12.87 

=  40 

'•25 

2.50 

3-75 

5.00 

6.25 

7.50 

8-75 

10.00 

11.25 

:45 

1.  11 

2.22 

3-33 

4-44 

5-55 

6.66 

7-77 

8.88 

IO.OO 

=  5° 

1.  00 

2.00 

3.00 

4.00 

5.00 

6.00 

7.00 

8.00 

9.00 

:55 

0.91 

1.82 

2-73 

3-64 

4-55 

S-46 

6.36 

7.27 

8.17 

:6o 

0.83 

1.67 

2.50 

3-33 

4.17 

5.00 

5-63 

6.67 

7.50 

1                i 

Steam-Boiler  Construction.     Walter  S.  Hutton.     London, 


1 32  MECHANICAL   DRAFT. 

evaporated  from  and  at  212°  per  pound  of  coal,  the  evaporation  per  square  foot 
of  heating  surface  has  been  calculated  for  different  rates  of  combustion  under 
different  surface  ratios.  Of  course  this  table  is  purely  theoretical,  and  applies 
only  when  the  efficiency  is  10  pounds,  but  it  indicates  relative  values. 

It  is  evident  that,  with  a  given  boiler,  it  is  a  comparatively  simple  matter  to 
so  change  the  setting  as  to  permit  of  the  use  of  grates  of  a  different  area,  thus 
altering  the  surface  ratio ;  and,  further,  that  a  reduction  of  grate  surface  with 
the  maintenance  of  the  same  rate  of  combustion  is  relatively  equivalent  to  an 
increase  in  the  heating  surface  without  change  of  the  original  grate.  A  reduc- 
tion of  grate  area,  without  corresponding  increase  in  the  rate  of  combustion, 
such  that  the  total  amount  of  fuel  consumed  remains  the  same,  must  of  neces- 
sity reduce  the  rate  of  evaporation  per  square  foot  of  heating  surface,  and  con- 
sequently the  capacity  of  the  boiler.  Under  such  conditions  the  evaporative 
efficiency  may  be  improved,  but  the  boiler  be  actually  incapable  of  performing 
its  previous  amount  of  work.  It  is  by  such  a  combination  of  circumstances 
that  the  unprincipled  advocates  of  certain  so-called  coal-saving  devices  have 
sometimes  been  able  to  show  an  increased  evaporation  per  pound  of  fuel,  which, 
while  satisfactory  in  itself,  was  not  secured  under  the  ordinary  working  condi- 
tions, and  might  possibly  have  been  as  easily  secured  without  the  device.  The 
capacity  may  be  maintained,  even  with  reduced  grate  area,  by  correspondingly 
increasing  the  rate  of  combustion.  The  limit  to  such  arrangements  is  naturally 
set  by  the  amount  of  fuel  which  can  be  fired  and  maintained  in  good  condition 
per  square  foot  of  grate ;  while  the  evaporative  capacity  of  the  heating  surface 
is  limited  by  the  ability  of  the  steam  bubbles  to  readily  escape  from  its  surface, 
which  ability  in  turn  is  largely  dependent  upon  the  arrangement  of  the  surface. 

The  possibilities  of  increased  capacity  are  evidenced  in  the  statement  of 
Mr.  W.  S.  Hutton,1  that  "with  an  efficient  system  of  forced  draft  i  indicated 
horse-power  may  generally  be  economically  developed  from  2  square  feet  of 
heating  surface  of  marine  return  tubular  boilers  fired  with  good  coal.  The  pro- 
portion of  heating  surface  to  fire-grate  surface  may  be  45  to  i.  Small  tubes  are 
more  effective  with  forced  draft  than  large  tubes,  and  the  smoke  tubes  may  be 
at  least  one-quarter  less  in  diameter  than  the  tubes  used  for  natural  draft." 

Economy  of  High.  Rates  of  Combustion.  —  It  has  been  shown  in  the  preced- 
ing chapter  that  with  a  constant  surface  ratio  the  evaporative  efficiency  of  the 
boiler  decreases  as  the  rate  of  combustion  is  increased.  But  an  increase  of 
the  rate,  while  the  surface  ratio  remains  the  same,  is  in  effect  an  increase  in 
the  total  quantity  of  coal  consumed.  The  more  the  coal  consumed  the  more 


Steam-Boiler  Construction.     Walter  S.  Hutton.     London,  1891. 


MECHANICAL    DRAFT.  133 

the  air  required ;  hence  the  logical  deduction,  substantiated  by  experience, 
that  the  efficiency  will  be  reduced.  For  the  larger  volume  of  gases  travelling 
at  higher  velocity  will  impart  relatively  less  heat  to  the  exposed  surfaces  and 
enter  the  chimney  at  a  higher  temperature.  But  in  practice  certain  factors  af- 
fect these  theoretical  considerations,  so  that,  as  evidenced  by  statements  which 
follow,  the  rate  of  combustion  upon  a  given  grate  may  be  greatly  increased 
without  adverse  economic  effect. 

It  is  to  be  noted  that  when  the  surface  ratio  is  constant  the  rate  of  combus- 
tion becomes  practically  a  direct  measure  of  the  total  amount  of  fuel  consumed. 
If,  however,  the  ratio  be  changed,  as  for  instance  by  reducing  the  grate  area, 
the  total  consumption  can  only  be  maintained  by  increasing  the  rate  of  combus- 
tion per  square  foot.  Under  this  condition  it  is  certainly  evident  that  at  least 
no  greater  amount  of  air  will  be  required  per  pound  of  coal.  In  fact,  experience 
shows  that  ordinarily  the  amount  of  air  required  will  actually  be  reduced.  This 
naturally  results  in  an  increase  in  the  efficiency.  Upon  this  fact  rests  one  of 
the  important  advantages  of  mechanical  draft,  for  by  its  means  may  be  pro- 
duced the  intensity  of  draft  requisite  to  high  combustion  rates. 

The  conditions  under  which  this  increased  efficiency  can  be  secured  must, 
however,  be  clearly  understood.  An  increase  in  the  rate  of  combustion,  when 
it  is  accompanied  by  a  greater  coal  consumption,  as  would  be  the  case  where 
the  surface  ratio  remains  constant,  is  not  always  conducive  to  economy.  But, 
when  the  total  amount  of  coal  consumed  remains  the  same,  and  the  increased 
combustion  rate  is  secured  by  a  reduction  of  grate  area  and  corresponding 
increase  in  the  surface  ratio,  a  higher  efficiency  is  the  natural  result.  This  is 
shown  by  Table  No.  73,  the  figures  in  which  give  relative  values. 

As  there  indicated,  the  efficiency  of  the  fuel,  or  the  water  evaporated  from 
and  at  212°  per  pound  of  coal,  is  10.23  pounds  for  a  stationary  boiler,  where  the 
rate  of  combustion  is  30  pounds  per  square  foot  of  grate  and  the  surface  ratio 
is  30.  This  is  equivalent  to  burning  30  -=-  30  =  i  pound  of  coal  per  square 
foot  of  heating  surface  per  hour.  If  the  surface  ratio  were  50,  and  the  coal 
consumption  per  square  foot  of  heating  surface  remained  the  same,- —  namely,  i 
pound, —  the  rate  per  square  foot  of  grate  would  be  50  x  1=50  pounds.  The 
table  shows  that  with  these  conditions  of  surface  ratio  and  rate  of  combustion 
the  evaporation  would  be  10.67  pounds,  an  increase  of  about  4  per  cent.  The 
high  rates  are  chosen  for  illustration  only  because  they  avoid  interpolation  in 
the  table;  but  the  same  principle  holds  throughout.  For  instance,  a  rate  of  25 
pounds  per  square  foot  of  grate,  with  a  ratio  of  50,  gives  about  9  per  cent  higher 
efficiency  than  a  rate  of  15  pounds  and  a  ratio  of  30,  although  the  coal  con- 
sumption per  square  foot  of  heating  surface  is  the  same. 


i34  MECHANICAL   DRAFT.  . 

The  principal  if  not  the  sole  cause  for  this  increase  in  efficiency  is  to  be 
found  in  the  decreased  supply  of  air  which  is  required  per  pound  of  coal  when 
the  rate  of  combustion  per  square  foot  of  grate  is  raised.  The  reason  of  the 
decreased  requirement  appears  evident  in  the  fact  that  the  higher  rate  of  com- 
bustion necessitates  a  deeper  fire,  and  that  the  air  supplied  is,  therefore,  com- 
pelled to  come  in  contact  with  a  greater  amount  of  fuel,  and  afforded  a  better 
opportunity  to  promote  perfect  combustion.  The  intensity  of  the  fire  is  in- 
creased, its  temperature  is  higher,  more  heat  is  radiated  to  the  exposed  boiler 
surfaces,  and  more  is  taken  up  by  the  gases.  Furthermore,  the  diminished 
superficial  area  of  the  grate  and  of  the  exposed  interstices  between  the  fuel 
necessitates  a  higher  velocity  to  secure  the  admission  of  a  given  volume  of  air. 
This  increased  velocity  in  turn  requires  greater  draft  or  air  pressure.  The 
volume  at  a  given  temperature  passing  through  the  coal  is  proportional  to  the 
velocity,  but  the  pressure  varies  as  the  square  of  the  velocity.  Therefore,  if  a 
given  grate  be  reduced  one-half,  and  the  rate  of  combustion  doubled,  so  as 
to  maintain  the  same  total  consumption,  the  same  volume  of  air  would  have 
to  travel  through  the  exposed  interstices  at  twice  the  velocity.  But  the  pres- 
sure or  vacuum  would  be  four  times  as  great,  and,  as  a  consequence,  the  air 
would  be  forced  or  drawn  into  spaces  between  the  fuel  which  it  could  not  reach 
under  lesser  impelling  force.  Much  more  intimate  contact  and  distribution  are 
the  results.  Less  free  oxygen  passes  through  the  fuel  bed  unconsumed,  and  for 
a  given  supply  of  air  a  higher  efficiency  of  the  fuel  is  attained.  But  the  high 
pressures  necessary  to  such  results  seldom  exist  or  are  attainable  where  a 
chimney  is  depended  upon  for  the  production  of  the  draft.  In  fact,  the  present 
rates  of  combustion  common  in  factory  practice  are  such,  largely  because  of 
the  inability  of  a  chimney  of  moderate  height  and  cost  to  economically  provide 
draft  of  sufficient  intensity  for  higher  rates.  For  this  reason  forced  or  mechan- 
ical draft  has  always  been  considered  in  a  sense  separate  and  apart  from 
chimney  draft,  and  until  recently  has  been  regarded  principally  in  the  capacity 
of  producing  draft  pressures  beyond  the  limits  of  the  ordinary  chimney.  Con- 
sequently, the  effects  of  increased  draft  and  higher  combustion  rates  have  Ween 
attributed  directly  to  the  use  of  mechanical  means ;  and,  in  fact,  experience  has 
shown  that  these  effects  can  be  economically  attained  only  by  the  employment 
of  such  means.  This  fact  it  is  the  object  of  this  work  to  establish  by  an  im- 
partial statement  of  such  experience. 

Thus  Rankine  wrote  nearly  forty  years  ago  that  "  in  furnaces  where  the 
draft  is  produced  by  means  of  a  blast  pipe,  like  those  of  locomotive  engines 
or  by  means  of  a  fan,  the  quantity  of  air  required  for  dilution,  although  it  has 
not  yet  been  exactly  ascertained,  is  certainly  much  less  than  that  which  is 


MECHANICAL   DRAFT. 


J35 


required  in  furnaces  with  chimney  drafts  ;  and  there  is  reason  to  believe  that 
on  an  average  it  may  be  estimated  at  about  one-half  of  the  air  required  for 
combustion." 

After  stating  that  a  "  high  [smoke]  pipe  increases  economy  of  combustion,  due 
to  more  energetic  combination  of  oxygen  and  fuel,  Engineer-in-Chief  Melville1 
states  that  '  this  has  been  repeatedly  shown  with  moderate  forced  draft.' " 

Hutton2  bases  his  estimate  of  increased  efficiency  with  mechanical  draft  upon 
the  decreased  amount  of  air  required  therewith,  and  the  resulting  higher  fur- 
nace temperature.  Accepting  24  pounds  of  air  as  necessary  per  pound  of  coal 
under  natural  draft,  he  shows  the  furnace  temperature  to  be  2,926°,  with  perfect 
combustion,  giving  an  efficiency  of  66  per  cent.  Accepting  18  pounds  as 
required  under  forced  draft,  the  furnace  temperature  is  shown  to  be  3,686°, 
and  the  efficiency  76  per  cent. 

He  states  that  "  more  complete  combustion,  giving  a  higher  temperature,  may 
be  obtained  in  a  furnace  with  forced  than  with  natural  draft.  The  heating  sur- 
faces of  the  boiler  are  also  more  efficient  because  there  is  a  greater  difference 
in  the  temperature  of  the  water  surface  and  fire  surface  of  the  plates  forming 
the  heating  surfaces.  As  the  rate  of  transfer  of  heat  varies  as  the  difference  in 
temperature  of  the  water  on  one  side  of  the  plate  and  that  of  the  fuel  gases  on 
the  other  side  ;  the  greater  the  difference,  the  greater  the  amount  of  heat  which 
will  pass  through  a  unit  of  heating  surface  in  a  given  time." 

This  subject  has  not  been  so  carefully  investigated  as  its  importance  war- 
rants, but  it  is  a  well-established  fact  that,  under  ordinary  conditions,  a 
decreased  amount  of  air  is  required  with  rapid  rates  of  combustion,  such  as 
obtain  in  mechanical  draft  practice.  Owing  to  the  influence  of  surface  ratios, 
kind  of  fuel,  construction  of  the  furnace,  and  the  like,  any  absolute  basis  of 
comparison  is,  however,  practically  impossible. 

But  the  importance  of  mechanical  draft  as  an  adjunct  to  economical  combus- 
tion is  appreciated  by  all  progressive  engineers.  Thus  Prof.  Carpenters  con- 
cisely and  conservatively  states  that  "  one  of  the  requisites  of  economy  that  is 
required  is  high  temperature  in  the  furnace.  This,  on  the  other  hand,  means  a 
small  supply  of  air,  but  an  amount  which  is  sufficient  to  support  combustion. 
To  secure  perfect  combustion  with  a  small  amount  of  air  requires  an  intimate 
distribution  of  the  air  and  fuel.  This  of  necessity  requires  a  strong  draft,  which 


1  Machinery  of  the  New  Vessels  of  the  United  States  Navy.      George  W.  Melville.     Trans- 
actions Society  of  Naval  Architects  and  Marine  Engineers.     1893. 

2  Steam-Boiler  Construction.     Walter  S.  Hutton.     London,  1891. 

3  Wastes  from  Boiler  Management.     R.  C.  Carpenter.     Machinery,  June,  1895. 


136  MECHANICAL   DRAFT. 

possibly  can  be  more  cheaply  produced  by  mechanical  means  than  by  heating 
the  chimney."  He  then  proceeds  to  show  by  the  results  of  an  actual  test  "that 
for  that  plant,  at  least,  a  gain  of  a  good  many  horse-power  would  have  been 
possible  by  the  substitution  of  mechanical  for  natural  draft ;  provided  the  heat 
discharged  from  the  flue  could  have  been  prevented  and  utilized." 

Recent  extended  experience  with  American  boilers  and  coals,  as  shown  by 
the  following  quotations,  points  clearly  to  the  substantial  maintenance  under 
proper  operation  of  the  evaporative  efficiency  with  widely  varying  rates  of  com- 
bustion. Mr.  F.  R.  Low,  in  the  recent  report  of  the  Committee  on  Data  of  the 
National  Electric  Light  Association,1  states :  "  I  have  plotted  the  water  evapo- 
rated per  hour  per  square  foot  of  heating  surface,  and  the  pounds  of  water 
evaporated  from  and  at  2 12°  per  pound  of  combustible,  as  determined  by  30 
different  tests  on  Babcock  &  Wilcox  boilers,  and  proved  that  practically  as  good 
results  are  obtained  at  over  5  pounds  per  square  foot  of  heating  surface  as  at 
I-75  pounds,  and  the  intermediate  tests  show  no  dependence  on  the  rate  of 
evaporation.  I  have  plotted  in  the  same  way  all  of  the  tests  of  which  I  could 
find  a  record,  and  in  this  wide  range  still  no  evidence  is  apparent  of  any  de- 
pendence of  the  boilers  represented  on  the  rate  of  evaporation  within  the  range 
covered.  This  means  that  the  variation  of  efficiency  of  boilers  within  the  range 
here  comprised  is  less  than  the  variations  due  to  different  firing,  etc. 

"  Wm.  H.  Bryan,  in  a  paper  read  before  the  Engineers'  Club  of  St.  Louis, 
gives  the  data  and  tests  in  which  a  battery  of  horizontal  tubular  boilers  were 
forced  to  more  than  double  their  rating  with  only  the  following  improvement  of 
their  efficiency :  [In  brief  the  tabulated  results  show  the  coal  per  square  foot  of 
grate  surface  to  range  between  18.074  and  43.68,  that  per  square  foot  of  heat- 
ing surface  between  0.332  and  0.803,  tne  water  evaporated  per  square  foot  of 
heating  surface  between  2.43  and  5.235,  and  the  evaporation  per  pound  of  com- 
bustible from  and  at  212°,  between  9.27  and  8.827.  In  percentage  of  rated 
capacity  the  range  is  between  100.2  and  219.83,  while  the  efficiency  percentage 
of  heat  utilized  varies  between  76.38  in  the  former  and  68.83  m  the  latter.] 
Here  is  a  battery  of  boilers  which  were  forced  to  nearly  double  their  rated 
capacity  with  a  decrease  of  only  6  per  cent  in  their  efficiency,  and  which  could 
doubtless  have  been  diminished  to  one-third  of  their  rated  capacity  with  a  less 
impairment  still.  In  other  words,  with  good  management  and  an  adaptation  to 
conditions,  these  boilers  would  have  taken  care  of  a  maximum  load  six  times  the 
minimum  without  suffering  extremely."  These  statements  are  not  to  be  over- 
looked when  considering  the  application  of  mechanical  draft. 


The  Engineering  Record.     New  York,  June  26,  1897. 


MECHANICAL   DRAFT. 


'37 


Clark1  states  that  "the  proportion  of  surplus  air  required  appears  to  diminish 
as  the  rate  of  combustion  and  the  general  temperature  in  the  furnace  are 
increased,"  and  presents  the  results  here  given  in  Table  No.  85  as  evidence  of 
the  truth  of  this  statement.  Although  the  surplus  air  with  the  Longridge  boiler 
appears  extremely  low,  yet  the  experiments  of  Whitham,  given  in  Table  No.  86, 
although  conducted  under  special  conditions,  tend  to  confirm  its  probability. 
Table  No.  85.  —  Surplus  Air  with  Different  Rates  of  Combustion. 


KIND  OF  BOILER. 

Coal  consumed  per  Square  Foot 
of  Grate  per  Hour.    Pounds. 

Surplus  Air.     Per  cent. 

Cornish          .         .         .         . 
Delabeche  and  Playfair 
Longridge      .         . 

2  to     4 
10  to  16 
20  and  upwards. 

100 
25  to  50 
9^ 

Although  it  is  commonly  accepted  that  the  air  supply  with  chimney  draft  is 
about  300  cubic  feet  (approximately  24  pounds  per  pound  of  coal  where  the 
combustion  rate  is  from  10  to  15),  yet  there  is  in  practice  a  wide  variation  from 
this  standard.  Thus  the  tests  of  Messrs.  Donkin  and  Kennedy,  already  pre- 
sented in  Table  No.  16,  show  a  range  between  17.3  pounds  and  39.2  pounds 
per  pound  of  coal ;  that  is,  an  excess  of  56  to  260  per  cent.  In  marine  practice, 
with  mechanical  draft,  a  combustion  rate  of  30  to  40  pounds  may  be  easily 
maintained  with  a  supply  of  225  cubic  feet  of  air  per  pound  of  coal.  Under 
the  influence  of  mechanical  draft,  whereby  the  volume  may  be  readily  con- 
trolled, the  air  supply  requisite  for  successful  combustion  is  much  reduced 
below  that  with  chimney  draft.  How  much  this  reduction  may  be  must  depend 
upon  the  conditions,  —  character  of  fuel,  rate  of  combustion,  etc. 

Table  No.  86.  —  Air  Supply  with  Different  Rates  of  Combustion  on  a  Wilkinson 
Automatic  Mechanical  Stoker. 


Buckwheat  Coal  burned 
per  Hour  per  Square  Foot 
of  Stoker  Grate. 

Air  Theoretically  Required 
to  burn  One  Pound  of. 
Buckwheat  Coal. 

Air  Actually  Supplied 
to  burn  One  Pound  of 
Buckwheat  Coal. 

Percentage  of  Excess  or 
Deficiency  of  Air  Supplied. 

Pounds.                                  Cubic  Feet.                                 Cubic  Feet. 

1 

12.0 

125 

232 

+  85.6 

1  8.0 

125- 

'57 

+  25.6 

25.2 

I25 

132 

+    5-6 

32-5 

I25 

123 

—    1.6 

4i-5 

125 

III                                                  —  II.  2 

45-4 

125                                                        III                                                  —  II.  2 

I 

The  Steam  Engine.     D.  K.  Clark.     London,  &c.,  1890. 


'38 


MECHANICAL    DRAFT. 


Greater  care  in  the  distribution  of  the  air  and  in  maintaining  the  condition  of 
the  fire  is  necessary  to  a  successful  reduction  in  the  air  supply.  This  is  very 
clearly  evidenced  by  the  remarkable  results  of  tests  by  Mr.  J.  M.  Whitham1 
upon  automatic  mechanical  stokers.  Those  relating  to  the  Wilkinson  stoker, 
under  different  rates  of  combustion,  are  presented  in  Table  No.  86.  It  is  to  be 
noted  that  there  is  an  excess  of  air  up  to  a  combustion  of  30  pounds  of  dry  coal 
per  hour  per  square  foot  of  grate.  An  evaporative  efficiency  of  11.69  pounds 
of  water  from  and  at  212°  per  pound  of  combustible  was  recorded  when  burn- 
ing 45.4  pounds,  while  the  table  shows  that  there  was  an  actual  deficiency  in 
the  air  supply.  The  natural  explanation  of  maintained  efficiency  with  this  air 
supply  is  to  be  found  in  the  construction  of  the  stoker.  It  consists  of  a  number 
of  hollow  grate  bars,  to  the  ends  of  which  the  air  is  admitted  and  through 
numerous  small  holes  in  the  upper  surfaces  of  which  it  escapes.  The  air  is 
thus  thoroughly  diffused  throughout  the  fire ;  there  is  practically  perfect  contact 
of  air  and  fuel,  and  consequent  combustion  with  the  minimum  of  waste  gases. 

The  deductions  of  Mr.  D.  K.  Clark,  already  given  in  the  preceding  chapter, 
regarding  the  relation  of  grate  area,  heating  surface,  water  and  fuel,  serve  to 
confirm  the  statement  that  an  increased  rate  of  combustion  does  not  entail 
decreased  efficiency  when  the  total  consumption  remains  constant.  Regardless 
of  any  reduction  in  the  air  supply  required  per  pound  of  coal  when  the  rate  is 
increased,  he  showed  by  the  results  of  extended  experiment2  upon  locomotive 
boilers  that  the  efficiency  remained  practically  constant  when  the  rate  of  com- 
bustion increased  and  the  total  evaporation  proceeded  in  the  ratio  of  the  square 
of  the  surface  ratio.  The  results,  with  values  calculated  therefrom,  are  pre- 
sented in  Table  No.  87.  It  is  to  be  noted  that  under  the  experimental  condi- 
tions the  capacity  of  the  group  of  boilers,  as  measured  in  pounds  of  water 

Table  No.  87.  —  Effects  of  Different  Surface  Ratios  and  Rates  of  Combustion. 


Designation 
Groups  of  Tests. 

Coke  Consumed 
per  Square  Foot  of 
Grate  per  Hour. 

Average  Ratio  of 
Heating  Surface  to 
Grate  Surface. 

Coke  Consumed 
per  Square  Foot 
of  Heating  Sur- 
face per  Hour. 

Water  Evaporated 
per  Square  Foot 
of  Heating  Sur- 
face per  Hour. 

Water  Evaporated 
per  Pound  of 
Coke. 

Pounds. 

Pounds. 

Pounds. 

Pounds. 

A 

42.7 

52 

0.82 

7-4 

9.0 

B 

55.0 

66 

0.83 

7.6 

9.1 

C 

86.0 

72 

1.19                      10.6 

8.9 

D 

126.0 

90 

1.40 

I2.S 

8.92 

1  Experiments  with  Automatic  Mechanical  Stokers.     J.  M.  Whitham.     Transactions  Ameri- 
can Society  of  Mechanical  Engineers,  Vol.  XVII. 

2  The  Steam  Engine.     D.  K.  Clark.     London,  &c.,  1890. 


MECHANICAL    DRAFT.  139 

evaporated  per  hour  per  square  foot  of  heating  surface,  increased  from  7.4  to 
12.5,  or  69  per  cent,  while  the  efficiency  remained  substantially  constant.  It  is 
a  perfectly  reasonable  inference  that  had  the  capacity  been  maintained  constant, 
the  efficiency  would  have  increased  with  the  rate  of  combustion. 

Further  and  more  direct  evidence  that  with  a  given  total  coal  consumption 
the  efficiency  of  the  boiler  rises  as  the  rate  of  combustion  is  increased,  is  pre- 
sented by  the  tests  of  M.  Burnat1  upon  a  French  boiler  with  grates  of  three 
different  areas.  The  principal  items  in  these  results  are  given  in  Table  No.  88, 

Table  No.  88.  —  Results  of  Performance  of  French  Boiler  with  Grates  of  Different 

Areas. 


Area  of  Grate. 

Air  at  62° 
Fahr.  per 

Average  Temperature. 

Coal  Consumed. 

Residue. 

Water  per 
Pound  of 

Pound  of 

Coal  from 

Square  Feet. 

Coal. 
Cubic  Feet. 

Feed  Water. 
Degrees. 

Gas  at 
Dampers. 
Degrees. 

Per  Hour. 
Pounds. 

Per  Hour  per 
Square  Foot 
of  Grate. 
Pounds. 

and  at  212° 
Per  cent,     j      Pounds. 

24.70                161 

I24o 

5760 

125 

5.28 

I6.5 

7.26 

12-37 

164 

124 

612                  127 

11.00 

I8.7 

7-54 

9-°3 

180 

II7 

570 

124 

14.74 

19.0                7.79 

which  shows  that  with  practically  the  same  quantity  of  coal  consumed  per  hour 
on  the  three  grates  the  efficiency  increased  as  the  grate  area  was  diminished 
in  the  ratios  of  7.26  pounds,  7.54  pounds  and  7.79  pounds  of  water  per  pound 
of  coal,  although  in  the  third  case  a  larger  supply  of  air  was  provided. 

Similar  confirmatory  results2  with  grates  6  feet  and  4  feet  long  are  presented 
in  Table  No.  89,  showing  that  higher  efficiency  and  rapidity  of  evaporation  were 
obtained  from  the  shorter  grate  with  about  equal  quantities  of  coal  per  hour. 

Table  No.  89.  —  Effect  of  Length  of  Grate  upon  Efficiency  and  Rapidity  of 
Evaporation. 


LENGTH  OF  GRATI 


4  Feet. 

6  Feet. 

Total  coal  per  hour      .         .         .         .         .         ... 
Coal  per  hour  per  square  foot  of  grate 
Water  at  2120  evaporated  per  pound  of  coal 

pounds, 
pounds, 
pounds, 

400 
14 
10.10 

414 

23 
10.91 

'  Bulletin  de  la  Societe  Industrielle  de  Mulhouse.  Vol. 

XXX.  i 

SSQ-OO. 

140 


MECHANICAL    DRAFT. 


The  inference  must  of  necessity  be  drawn  from  the  preceding  that  an  increase 
in  the  surface  ratio  and  a  corresponding  increase  in  the  rate  of  combustion  will 
under  proper  conditions  result  in  raising  the  efficiency.  Under  the  present 
conditions  of  boiler  design  and  the  arrangements  existing  in  most  boiler  plants, 
the  simplest  means  of  securing  the  desired  results  would  appear  to  lie  in  the 
introduction  of  feed-water,  or  air  heaters,  or  similar  devices  of  such  proportions 
that  the  gases  resulting  from  the  more  rapid  combustion  produced  by  mechanical 
means  will  be  prevented  from  passing  to  the  fan  at  too  high  a  temperature. 

Thickness  of  Fire.  —  It  is  commonly  accepted  that  to  economically  burn  an 
increased  quantity  of  coal  per  square  foot  per  hour  it  is  necessary  to  increase 
the  thickness  of  the  layer  of  fuel.  Comparative  tests  with  different  thicknesses 
of  fire  in  a  marine  boiler,  by  Messrs.  Richardson  and  Fletcher,1  showed,  per 
Table  No.  90,  that  the  efficiency  increased  with  the  thickness  of  the  fire. 

Table  No.  90.  — Efficiency  of  Thick  Fires. 


ITEMS. 

THICKNESS  OF  FIRE. 

9-inch. 

i2-inch. 

i4-inch. 

Coal  consumed  per  square  foot  of  grate  per  hour,         pounds, 
Water  evaporated  per  pound  of  coal,  as  supplied       j       uncj<; 
at  2120,                                                                           \  P 

27 
10.77 

27 
11.23 

27 

"•54 

There  are  conditions,  however,  where  the  thinner  fire  may  prove  the  more 
economical,  as  where  the  rate  of  combustion  is  such  that  a  heavy  fire,  fed  at 
long  intervals  and  given  but  little  attention  is  compared  with  a  thinner  fire  fed 
more  frequently  and  run  under  better  management.  Evidently,  the  stronger 
draft  is  required  with  the  thicker  fire ;  but  this,  as  has  already  been  pointed  out, 
should  be  an  element  in  the  increased  efficiency  because  of  the  greater  pressure 
which  causes  more  intimate  contact  of  air  and  fuel,  as  with  mechanical  draft. 

As  stated  by  Mr.  W.  S.  Hutton,2  "  A  thick  fire  is  necessary  for  economical 
combustion  with  forced  draft.  It  should  not,  in  a  general  way,  be  less  than  10 
inches  thick,  and  it  should  not  be  allowed  to  burn  down  to  a  less  thickness  than  7 
inches  before  stoking.  A  thin  fire  causes  loss  from  the  entrance  through  the  fuel 
of  an  excessive  supply  of  air.  The  stronger  the  draft  the  thicker  must  the  fire  be." 


1  Report  on  the  Boiler  and  Smoke  Prevention  Trials,  conducted  at  Wigan,  1869. 

2  Steam-Boiler  Construction.     Walter  S.  Hutton.     London,  1891. 


CHAPTER   VIII. 
DRAFT. 

Definition.  —  More  or  less  confusion  exists  in  the  use  of  the  term  "draft"  in 
boiler  practice  because  of  its  double  meaning.  As  usually  employed,  it  refers 
to  the  difference  in  pressure  between  the  external  air  and  the  gases  as  they 
leave  the  boiler ;  although,  as  related  to  the  combustion  of  the  fuel,  it  should 
properly  apply  to  the  difference  between  the  under-  and  over-grate  pressures.  In 
either  application  it  indicates  the  intensity  or  force  of  the  draft,  and  is  generally 
measured  in  inches  of  water  by  means  of  a  draft  gauge.  The  term  "  draft "  is, 
however,  sometimes  employed  as  a  measure  of  the  volume  or  weight  of  the 
gases  passing  through  the  fire.  As  the  readings  of  a  draft  gauge  give  no  direct 
indication  of  their  volume,  the  quantity  of  air  or  gases  must  be  determined  by 
other  means. 

In  the  case  of  a  chimney,  the  maximum  intensity  of  draft  exists  only  with  the 
maximum  temperature  of  the  gases  ;  but  after  the  temperature  reaches  about 
600°  Fahr.  their  density  decreases  more  rapidly  than  their  velocity  increases, 
so  that  the  weight  of  air  supplied  is  a  maximum  at  about  this  temperature.  As 
the  draft  is  almost  universally  measured  by  the  difference  in  pressure,  expressed 
either  in  inches  of  water  or  in  weight  per  unit  of  area,  the  term  will  be  here  em- 
ployed as  indicating  the  intensity  or  force  with  which  it  acts.  This  difference  in 
pressure,  whether  it  be  the  result  of  creating  a  plenum  in  the  ashpit  or  a  par- 
tial vacuum  in  the  boiler  furnace,  is  always  necessary  to  produce  the  flow  of  air 
through  the  fuel  whereby  combustion  is  maintained.  It  is  evident,  therefore,  that 
the  draft  or  pressure  difference  and  the  velocity  or  air  flow  are  interdependent. 

Relation  of  Pressure  and  Velocity.  —  As  the  laws  which  govern  the  move- 
ment of  gases  are  the  same  as  those  which  apply  to  liquids  in  motion,  their 
application  can  be  most  readily  illustrated  by  means  of  a  liquid,  which  has 
visible  substance.  If  a  vessel  with  vertical  sides,  as  indicated  in  Fig.  4,  be 
filled  with  water  at  50°  Fahr.  to  the  level  A,  the  total  pressure  upon  the  bot- 
toms will  be  equal  to  the  weight  of  the  entire  quantity  of  water.  If  the  area 
of  the  base  be  100  square  inches,  and  the  total  weight  of  water  be  1,500 

pounds,  the  pressure  per  square  inch  will  be  — — —  =  15  pounds.     This  indi- 


142 


MECHANICAL   DRAFT. 


cates  that  each  column  of  water  having  i  square  inch  for  its  base,  and  the 
distance  AB  =  h  for  its  height,  weighs  1.5  pounds.  As  the  weight  of  water 
per  cubic  foot  at  50°  is  62.409  pounds,  and  consequently  0.0361  per  cubic  inch, 


it  is  also  evident  that  the  distance  AB  must  be 


0.036: 


=  415.3  inches  =  34.6 


feet.  This  depth  of  water,  7z,  producing  the  given  pressure  per  square  inch,  is 
known  as  the  total  head,  and  in  this  case  is  also  the  hydrostatic,  or  pressure 
head.  Obviously  the  pressure  exerted  is  directly  proportional 
to  the  head  or  depth  of  water.  For  the  cross-section  of  the 
vessel  remaining  constant,  any  change  in  the  depth  of 
the  water  must  result  in  a  coincident  change  in  the 
total  weight  of  water  which  presses  upon  the  bot- 
tom. 

If,  now,  a  pipe  C  be  inserted  in  the  side 
of  the  vessel  at  such  a  height  above 
the  base  that  the  distance  hI  is  25 
feet,  it  is  evident  that  the  pres- 
sure per  square  inch    of    cross- 
section  of  the  vessel  at  this  point 

will   be  — ^   of   that   upon    the 
34-6 

base;  that  is,  — —  x  15  =  10.85  pounds.      In  other  words,  it 

34-6 

will  be  equivalent  to  the  head  of  water  multiplied  by  its  density,   as   clearly 
shown  by  the  equations,  — 

-I 


B 


p  =  hd      and 


In  which  /  =  pressure. 
h  =  head. 
d  =  density. 

This  pressure  is  transmitted  to  the  water  in  the  end  of  the  pipe  at  its  junction 
with  the  vessel.  If,  now,  the  vessel  be  arranged  to  receive  a  continual  supply 
of  water  through  D,  such  that  its  level  A  will  be  kept  constant,  notwithstand- 
ing the  outward  flow  through  the  pipe  C,  the  head  /it  will  continue  the  same,  as 
will  also  the  pressure  which  is  exerted  upon  the  water  in  the  end  of  the  pipe. 
By  the  action  of  gravity  the  water  seeks  to  escape  through  the  pipe,  and  its 
effect  or  pressure  is  exactly  proportional  to  the  head  or  depth  h^. 

If  there  were  no  friction  whatever  in  the  pipe  the  total  head  would  be  rendered 
effective  for  producing  flow,  and  the  speed  or  velocity  of  that  flow  would  be 


MECHANICAL   DRAFT.  143 

exactly  that  which  would  finally  result  if  a  body  under  the  action  of  gravity  had 
freely  fallen  the  distance  measured  by  the  head  h.  Therefore,  its  velocity  is 
determinable  by  the  well-known  formula  for  falling  bodies,  viz.  :  — 


In  which  i>  =  velocity  in  feet  per  second. 
h  =  the  head  in  feet. 
g  =  acceleration  due  to  gravity  =32.16. 

As  it  has  already  been  shown  that  //  =  —  ,  this  formula,  as  applied  to  the  veloc- 
ity of  movement  of  fluids,  may  take  the  form  — 


In  practice  the  walls  of  the  pipe  under  consideration  would  restrict  the 
freedom  of  flow,  and  a  portion  of  the  total  head  or  pressure  would  have  to  be 
expended  in  overcoming  the  resistance.  This  resistance  is  naturally  greatest  at 
the  part  of  the  pipe  farthest  removed  from  the  outlet,  for  the  freedom  of  flow 
increases  as  the  water  nears  the  end  of  the  pipe  where  there  is  no  resistance  ; 
the  atmospheric  pressure  at  this  point  being  balanced  by  that  upon  the  upper 
surface  of  the  water  in  the  vessel.  If  the  pipe  should  be  provided  with  a  series 
of  small  open-topped  gauges,  as  shown,  they  would  respectively  indicate,  by  the 
heights  of  water  within  them,  the  resistances  which  exist  at  the  different 
points.  The  decreasing  heights  of  these  columns  as  they  approach  the  end  of 
the  pipe  is  evidence  of  the  decreasing  resistance.  The  regularity  of  this  fall  is 
indicated  by  the  dotted  line  EF,  while  the  fall  in  the  total  head  available  at  each 
of  these  points  is  represented  by  the  line  GH.  Since  these  lines  are  parallel, 
the  vertical  distance  between  them,  which  represents  the  portion  of  the  head 
utilized  for  producing  velocity,  is  evidently  constant. 

As  the  total  head  represents  the  sole  means  of  producing  movement  of  the 
water,  whatever  portion  of  this  head  is  used  for  overcoming  resistance  reduces 
by  just  so  much  the  amount  that  remains  available  for  the  production  of  veloc- 
ity. That  portion  of  the  total  head  which  is  thus  employed  to  overcome  resist- 
ance is  known  as  the  pressure  head,  while  that  remaining  and  utilized  for  the 
production  of  velocity  is  designated  as  the  velocity  head, 

Evidently,  if  the  pipe  be  of  uniform  diameter  and  the  water  be  considered 
non-compressible,  the  velocity  must  be  uniform,  and  hence  the  velocity  or  pres- 
sure expended  for  producing  that  velocity  must  also  be  uniform.  This  confirms 
the  evidence  of  the  parallel  dotted  lines  in  the  figure. 


i44  MECHANICAL   DRAFT. 

With  a  constant  total  head  any  increase  in  the  length  of  the  pipe  naturally 
increases  the  pressure  head  and  consequently  reduces  the  velocity  head.  If, 
however,  the  pipe  be  entirely  removed,  leaving  only  the  orifice,  the  pressure 
head  will  be  practically  eliminated  and  the  total  head  will  become  the  velocity 
head.  The  actual  velocity  through  the  opening  will  be  very  slightly  less  than 
that  calculated  by  the  formula,  owing  to  the  slight  friction  of  the  water  in  pass- 
ing through.  But  the  volume  passing  through  an  opening  in  a  flat  plate  will  be 
considerably  less  than  that  which  would  be  calculated  by  multiplying  the  area 
of  the  opening  by  the  velocity,  or  rate  of  flow.  This  is  due  to  the  fact  that  the 
stream  contracts  as  it  leaves  the  opening,  so  that  its  minimum  area,  which  is  at 
a  short  distance  from  the  orifice,  becomes  only  about  two-thirds  of  the  area  of 
the  opening.  The  effect  of  different  forms  of  opening  will  be  discussed  in 
their  relation  to  the  efflux  of  air. 

Efflux  of  Air.  —  As  the  pressure  is  dependent  upon  both  the  height  and  the 
density  of  the  fluid,  it  is  evident  that  for  a  given  pressure  the  less  the  density 
the  greater  the  height  of  the  column.  But  the  law  of  falling  bodies  recognizes 
the  fact  that  it  is  the  distance  fallen  through  and  not  the  weight  of  the  body 
that  determines  its  velocity.  Therefore,  the  less  dense  a  body  the  higher  the 
column  required  to  produce  a  given  pressure  and  the  greater  the  velocity  of  dis- 
charge. From  this  it  is  evident  that  the  velocity  of  a  gas  issuing  under  a  given 
pressure  would  be  greater  than  that  of  a  liquid  under  the  same  conditions. 
And  conversely,  the  more  dense  the  fluid  issuing  at  a  given  velocity  the  greater 
must  have  been  the  pressure  to  produce  that  velocity. 

In  the  case  of  a  liquid,  the  atmospheric  pressure  upon  the  inlet  and  outlet  of 
a  containing  vessel  is  balanced  and  the  actual  height  or  head  may  be  actually 
measured.  But  air  is  invisible,  and  there  is  no  tangible  distinction  in  substance 
between  that  producing  the  pressure  and  that  constituting  the  surrounding 
atmosphere. 

The  pressure  of  the  atmosphere  is  due  to  the  weight  of  the  air,  and,  for  any 
area,  is  to  be  measured  by  the  weight  of  a  column  of  air  having  the  given  area 
as  a  base  and  a  height  equal  to  that  of  the  atmosphere.  But  this  height  can- 
not be  accurately  determined,  and,  furthermore,  the  density  of  the  air  decreases 
in  geometric  ratio  as  the  distance  from  the  earth  increases.  For  the  purposes 
of  calculation,  however,  the  practical  equivalent  of  such  a  column  may  be  deter- 
mined by  assuming  the  .air  to  be  of  uniform  density  throughout  and  the  column 
of  such  a  height  as  to  weigh  the  same  and  to  produce  the  same  effective  pres- 
sure per  unit  of  area. 

Under  the  standard  conditions  of  barometric  pressure  of  29.921  inches,  the 
atmospheric  pressure  is  14.69  pounds  per  square  inch,  or  2,115.36  pounds  per 


MECHANICAL   DRAFT.  145 

square  foot.     At  this  pressure  a  cubic  foot  of  dry  air  at  50°  has  a  density  of 

0.077884  pounds.      Consequently  a  homogeneous   column  2>        '^.     =  27,160 

0.07  7004 

feet  high,  having  a  base  of  one  square  foot  area,  would  weigh  2,115.36  pounds 
and  exert  this  pressure  upon  the  given  area. 

If  air  under  this  head  were  to  be  allowed  to  flow  freely  into  a  vacuum,  the 
velocity,  per  the  formula,  would  be  — 

''  =  */  2g  X   27,160 


x  27,160 
=        1,321.7  feet  per  second. 

In  the  case  of  air  flowing  into  a  vacuum  the  total  head  is  actual,  although 
here  reduced  for  simplicity  to  that  of  a  homogeneous  column.  But  under  any 
other  conditions  both  the  pressure  head  and  the  velocity  head  are  purely  ideal. 
The  fact  that  a  given  air  pressure  exists  in  a  reservoir  does  not  indicate  the 
existence  of  an  actual  column  of  air  of  known  density.  But  the  height  of  such 
an  ideal  column  is  readily  determinable  by  calculation  from  the  simple  equa- 
tion, — 

*-.! 

and  it  is  this  ideal  height  that  is  used  in  calculation. 

If,  as  is  frequently  the  case,  the  pressure  is  expressed  in  inches  of  water,  as 
indicated  by  the  balanced  height  of  a  column  of  that  liquid  in  a  water  gauge, 
this  may  be  readily  transformed  into  the  height  of  the  equivalent  column  of  air. 
Thus,  if  the  pressure  difference  in  inches  of  water  be  represented  by  ff,  and 
the  equivalent  head  of  air  in  feet  by  7i,  the  value  of  //  will  be,  — 

density  of  water  x  H 

12  x  density  of  air 
then  at  a  temperature  of  50°  Fahr. — 

62.409  x  H 

~    12   X  0.077884 
=   66.77^ 

If  this  value  be  employed  in  the  formula  for  velocity,  it  becomes  — 


v  =  V  64.3-2  x  66.77^ 
=  65.5  V^ff 

from  which  may  be  approximately  determined  the  velocity  of  efflux  of  air  under 
any  given  pressure  difference  expressed  in  inches  of  water.     As  the  value  of  H 


146  MECHANICAL   DRAFT. 

is  dependent  upon  the  temperature  of  both  air  and  water,  more  particularly  the 
former,  it  is  evident  that  the  value  of  the  constant  applies  only  under  the  stated 
conditions ;  and  the  value  of  v  must  be  considered  as  only  approximate  where 
the  formula  is  employed  under  other  conditions  without  suitable  corrections 
therefor. 

But  for  more  refined  calculation  additional  factors  must  be  taken  into  con- 
sideration. For  simplicity  the  atmospheric  pressure  and  humidity  may  be  con- 
sidered constant,  and  differences  of  pressure  rather  than  absolute  pressures  are 
employed.  In  the  case  of  air,  which  is  compressible  by  pressure  and  expansible 
by  heat,  the  density  must  vary  with  the  pressure ;  and  a  change  in  the  tempera- 
ture may  have  a  most  important  influence.  The  effect  of  increased  density, 
which  may  be  produced  by  the  pressure,  is  to  decrease  the  ideal  velocity  head. 

For  if  h  =  ~,  it  is  evident  that  an  increase  in  ^/must  for  a  given  pressure  reduce 

the  value  of-//.  A  similar  influence  is  exerted  by  the  temperature,  for  by  an 
increase  in  temperature  the  density  is  decreased,  and  hence  the  value  of  h  is 
increased.  The  velocity  being  dependent  solely  upon  the  ideal  head,  it  is  of 
the  utmost  importance  that  comparisons  be  reduced  to  the  same  conditions  of 
pressure,  temperature  and  density. 

If  the  pressure  is  to  be  expressed  in  ounces  per  square  inch,  which  is  readily 
reducible  to  inches  of  water,  the  formula  — 


-ft 


when  applied  under  the  conditions  of  — 

g  =  acceleration  due  to  gravity  =  32.16, 
p  =  pressure  in  ounces  per  square  inch, 

d=  density  or  weight  of  one  cubic  foot  of  dry  air  at  50°  tempera- 
ture, and  under  atmospheric  pressure  =  0.077884  pounds, 
becomes  — 


64-32  " 


16  x  0.077884  x  2'35 


235 

In  this  form  it  is  evident  that  the  density  varies  with  the  pressure,  for/  being 
expressed  in  ounces,  and  the  atmospheric  pressure  of  14.69  pounds  being 
equivalent  to  235  ounces,  the  density  which  exists  at  any  given  pressure  p  is 

0.0/7884  x  2**+p 


MECHANICAL   DRAFT.  147 

The  formula  reduces  to  — 


-i 
-j 


235 


0.077884 


1,746,659  x/ 

235  +/ 

The  formula,  when  the  pressure  is  expressed  in  inches  of  water,  and  allowance 
is  made  for  the  increased  density  of  the  air  due  to  the  pressure,  takes  the  form  — 


J  i, 746,659  x  h 
406.7  +  h 


Both  formulae  thus  take  into  account  the  compression  of  the  air  due  to  its 
pressure,  but  make  no  allowance  for  change  of  temperature  during  discharge. 

From  these  two  basis  formulae  the  velocities  given  in  Tables  Nos.  91  tand  92 
have  been  calculated ;  the  pressures  being  expressed  in  ounces  per  square  inch 
in  the  former  and  in  inches  of  water  in  the  latter.  In  both  tables  the  air  is 
assumed  to  be  dry  and  of  a  temperature  of  50°  Fahr.  As  it  has  already  been 
shown  that  the  velocity  varies  as  the  square  root  of  the  head,  and  that  the  head 

h  —  — ,  it  must  be  evident  that  any  decrease  in  the  density  of  the  air  due  to  a 

higher  temperature  must  for  the  same  pressure  increase  the  value  of  ft,  and  con- 
sequently increase  the  velocity.  Furthermore,  if  the  velocity  and  consequently 
the  head  are  to  be  maintained  constant  under  varying  temperatures,  the  pressure 
must  decrease  proportionately  to  the  density  as  affected  by  the  temperature. 

In  considerations  of  density  the  absolute  temperatures  only  are  concerned. 
Thus,  the  absolute  temperature  at  50°  Fahr.  is  461  -j-  50  =511°,  while  that  at, 
say,  550°  Fahr.  is  461  -(-  550  =  i,on.  Hence  the  relative  density  at  550°  Fahr. 

is  -SiL.  =  0.5054,  and  consequently  the  relative  volume  of  the  same  weight  of 

air,  expanded  by  the  increase  in  temperature,  is  —    —  =  1.978. 

The  relative  values  thus  calculated  have  been  incorporated  in  the  second  and 
third  columns  of  Table  No.  93.  From  the  second  column  it  is  evident  that  if 
the 'velocity  with  which  air  at  50°,  under  a  pressure  of  2^  ounces  per  square 
inch,  issues  from  an  orifice  be  8,135.7  feet  per  minute,  as  per  Table  No.  91,  then 
that  at  which  it  issues  under  the  same  pressure,  but  at  a  temperature  of  150°, 
will  be  1.09  X  8,135.7  =  8,867.9  feet.  Column  3  shows,  on  the  other  hand,  that 
if  a  velocity  of  8,867.9  feet  Per  minute  be  observed  under  the  above  conditions, 
it  must  have  been  due  to  a  pressure  of  0.84  x  2.5  =2.1  ounces  per  square  inch. 


MECHANICAL    DRAFT. 


Table  No.  91. — Velocity  Created,  Volume  Discharged  and    Horse-Power  Required 

when  Air  under  a  Given  Pressure  in  Ounces  per  Square  Inch  is 

Allowed  to  Escape  into  the  Atmosphere. 


Pressure  in  Ounces. 
Per  Square  Inch. 

Velocity  of  Dry  Air  at  50°  Temperature 
Fahr.  Escaping  into  the  Atmosphere  through 
any  Shaped  Orifice  in  any  Pipe  or  Reservoir  in 
which  the  Given  Pressure  is  Maintained. 

Volume  of  Air  in 
Cubic  Feet  which  may 
be  Discharged  in  One 
Minute  through  an 
Orifice  having  an 
Effective  Area  of 
Discharge  of 
One  Square  Inch. 

Horse-Power 
required  to  move  the 
Given  Volume  of  Air 
under  the  Given 
Condit.ons  of 
Discharge. 

In  Feet  per  Second. 

In  Feet  per  Minute. 

% 

30-47 

1,828.4 

12.69 

0.00043 

X. 

43.08 

2,585.0 

T7-95 

0.00122 

H 

52-75 

3.I65.I 

21.98 

0.00225 

% 

60.90 

3.653-8 

25-37 

0.00346 

X 

68.07 

4,084.0 

28.36 

0.00483 

X 

74-54 

4,472.6 

31.06 

0.00635 

H 

80.50 

4,829.7                               33.54 

0.00800 

i 

86.03 

5,161.7                               35.85 

0.00978 

i* 

91.22 

5.473-4 

38.01 

0.01166 

*x 

96.13 

5,768.0                         40.06 

0.01366 

1/8 

100.80 

6,047.9                         42.00 

0.01575 

i# 

105.25 

6,315.2                         43.86 

0.01794 

'# 

109.52 

6,57i-3 

45-63 

O.O2O22 

i# 

113.64 

6,817.6 

47-34 

0.02260 

i# 

117.58 

7,o55-o 

49.00 

0.02505 

2 

121.41 

7,284-4 

50-59 

0.02759 

*X 

125.11 

7,506.7 

52-13 

0.03021 

2X 

128.70 

7,722.2 

53-63 

0.03291 

2/8 

132.20 

7,931-8 

55.08 

0.03568 

*# 

135-59 

8,135-7 

56.50 

0.03852 

2^5 

138.91 

8,334-4 

57.88 

0.04144 

2^ 

142.14 

8,528.3 

59-22 

0.04442 

2^ 

145.29 

8,717.6 

60.54 

0.04747 

3 

148.38 

8,902.8 

61.83 

0.05058 

3/8 

151.40 

9,084.0 

63.08 

0-05376 

3X 

I54-36 

9,261.5 

64.32 

O.0570I 

3/8 

157.26 

9,435-4 

65.52 

0.06031 

3^ 

160.10 

9,606.1 

66.71 

0.06368 

3# 

162.89 

9,773-3 

67.87 

0.06710 

3X 

165-63 

9,938.0 

69.01 

0.07058 

3^ 

168.33 

10,099.6 

70.14 

0.07412 

MECHANICAL    DRAFT. 


Table  No.  91.  —  Velocity,  Volume  and  Horse-Power,  &c.  —  Concluded. 


I49 


Pressure. 

Velocity  per  Second. 

Velocity  per  Minute. 

Volume. 

Horse-Power. 

4 

170.98 

10,258.6 

71.24 

0.07771 

4X 

176.15 

10,568.8 

73-39 

0.08507 

4X 

181.16 

10,869.5 

75-48 

0.09264 

4^ 

186.03 

11,161.5 

77-51 

0.1004 

5 

190.76 

",445-5 

79-48 

0.1084 

5* 

1  95-37 

11,722.0 

81.40 

0.1166 

M 

199.86 

11,991.5 

.   83.24 

0.1249 

& 

204.25 

12,254.8 

85.10 

°-I335 

6 

208.53 

12,511.9 

86.89 

0.1422 

6^ 

216.82 

13,009.3 

90-34 

0.1602 

7 

224.77 

13,486.4 

93-66 

0.1788 

?X 

232.42 

13,945-4 

96.84 

0.1981 

8 

239.80 

14,387-9 

99.92 

0.2180 

S>^ 

346.92 

14,815.4 

102.88 

0.2385 

9 

253-83 

15,229.6 

105.76 

0.2596 

9^ 

260.52 

15,631.0 

108.55 

0.2812 

10 

267.00 

16,020.4 

111.25 

0.3034 

io# 

273-32 

16,399-3 

113.88 

0.3261 

II 

279.70 

16,768.1 

116.45 

0-3493 

II# 

285.46 

17,127.6 

118.94 

o-3730 

12 

291.30 

17,478.2 

121.38 

0.3972 

l*# 

297.01 

17,820.6 

1  23-7  5 

0.4219 

13 

302.59 

18,155.2 

126.06 

0.4470 

*3# 

308.04 

18,482.4 

128.35 

0.4726 

M 

3I3-38 

18,802.7 

130-57 

0.4986 

MX 

318.61 

19,116.3 

!32-75 

0.5250 

is 

323-73 

19,423.6 

134.89 

0.5518 

»SJ« 

328.7.5 

19,725.0 

136-98 

0.5791 

16 

333-68 

20,020.7 

I39-03 

0.6067 

'i6# 

338-5I 

20,310.8 

141.05 

0.6347 

i? 

343-26 

20,595.8 

143-03 

0.6631 

I7# 

347-93 

20,875.8 

144.97 

0.6919 

18 

352-52 

21,151.0 

146.88 

0.7211 

i8# 

357-03 

21,421.6 

148.76 

0-7506 

r9 

361.46 

21,687.8 

150.61 

0.7804. 

19^ 

365-83 

21,949-7 

1  52-43 

0.8107 

20 

370-I3 

22,207.5 

154.22 

0.8412 

MECHANICAL   DRAFT. 


Table  No.  92. —  Velocity  Created  when  Air  under  a  Given  Pressure  in  Inches  of 
'Water  is  Allowed  to  Escape  into  the  Atmosphere. 


Pressure 
in  Inches  of 
Water,  per 
Square  Inch. 

Velocity  of  Dry  Air  at  50°  Temperature 
Escaping  into  the  Atmosphere  through  any 
Shaped  Orifice  in  any  Pipe  or  Reservoir  in 
which  the  Given  Pressure  is  Maintained. 

Pressure 
in  Inches  of 
Water,  per 
Square  Inch. 

Velocity  of  Dry  Air  at  50°  Temperature 
Escaping  into  the  Atmosphere  through  any 
Shaped  Orifice  in  any  Pipe  or  Reservoir  in 
which  the  Given  Pressure  i»  Maintained. 

In  Feet 
per  Second. 

In  Feet 
per  Minute. 

In  Feet 
per  Second. 

In  Feet 
per  Minute. 

O.I 

20.72 

1.243-3 

2.6 

'05-33 

6,320.0 

0.2 

29.30 

1,758.0 

2.7 

107-33 

6,439-7 

o-3 

35-84 

2,150.4 

2.8 

109.28 

6,557.0 

0.4 

41-43 

2,485.6 

2.9 

III.  21 

6,672.3 

0.5 

46.31 

2,778.7 

3-° 

113.09 

6,785-5 

0.6 

50-73 

3.043-5 

3-i 

"4-95 

6,896.8 

0.7 

54-78 

3,287.0 

3-2 

116.77 

7,006.3 

0.8 

58.56 

3.5I3-5 

3-3 

118.57 

7,114.1 

0.9 

62.10 

3,726.1 

3-4 

120.34 

7,220.2 

1.0 

6545 

3'927-2 

3-5 

122.08 

7,324-7 

I.I 

68.64 

4,118.4    > 

3-6 

123.80 

7,427-7 

1.2 

71.68 

4,301.0 

3-7 

125.49 

7.529-3 

i-3 

74.60 

4,476.1 

3-8 

127.16 

7,629.4 

1.4 

77-41 

4,644.5 

3-9 

128.80 

7,728.2 

1-5 

80.  1  2 

4,806.9 

4.0 

i30-43 

7,825.7 

1.6 

82.73 

4.963-9 

4-25 

134.40 

8,064.1 

i-7 

85.27 

5,Il6.I 

4-5 

138.26 

8,295.4 

1.8 

87-73 

5.263.7 

4-75 

142.00 

8,520.1 

1.9 

90.12 

5.407-3 

5 

145.65                    8,738.8 

2.0 

92-45 

5.547-1 

5-25 

149.20 

8,951.8 

2.1 

94-72 

5,683.4 

5-5 

152.66 

9,159-7 

2.2 

96.94 

5,816.5 

5-75 

156.05 

9,362.8 

2-3 

99-11 

5,9464 

6 

1  59-35 

9,561.2 

2-4 

101.23 

6,073.6 

6.25 

162.59 

9.755-4 

2-5 

103.30 

6,198.1 

6.50 

165.76 

9,945.8 

In  the  preceding  discussion,  pressures  have  been  considered  to  be  above  the 
atmosphere ;  but  in  all  cases  pressure  differences  only  have  been  expressed. 
The  results  apply  as  accurately  to  pressures  below  the  atmosphere,  and  practi- 
cally may  be  so  applied ;  for  there  is  no  appreciable  difference,  for  instance, 
between  the  velocity  of  air  issuing  into  the  atmosphere  from  a  reservoir  under 
4  ounces  pressure  per  square  inch,  and  that  passing  from  the  atmosphere  into 
a  reservoir  in  which  a  vacuum  of  4  ounces  below  the  atmosphere  is  maintained. 


MECHANICAL   DRAFT.  151 

Influence  of  Form  of  Orifice.  - —  The  form  of  the  orifice  through  which  the  air 
passes  under  pressure  has  practically  no  effect  upon  the  velocity.  But  the  vol- 
ume of  air  discharged  is  largely  dependent  upon  the  character  of  the  opening. 
As  already  stated,  the  stream  of  any  fluid  escaping  through  an  orifice  in  a  thin 
plate  is  reduced  in  size  just  beyond  the  opening.  This  is  due  to  the  fact  that 
the  direction  of  the  molecules  of  the  fluid  is  changed  in  passing  through  the 
orifice.  Experiments  have  shown  that  this  vena  contracta,  or  contracted  vein,  at 
a  distance  from  the  orifice  equal  to  half  its  diameter,  experiences  its  maximum 
contraction,  and  that  in  the  case  of  water  its  diameter  is  about  0.8  of  that  of 
the  orifice.  The  orifice  being  round,  the  area  of  the  stream  varies  as  the  square 
of  the  diameter;  hence  the  area  becomes  o.82  =  0.64  of  that  of  the  orifice. 
This  quantity  is  known  as  the  coefficient  of  contraction.  The  coefficient  of  efflux 
is  the  product  of  the  coefficient  of  contraction  and  the  coefficient  of  velocity. 
The  latter  quantity,  in  the  case  of  water  flowing  through  a  round  orifice,  is  0.97 
without  appreciable  error,  and  may  be  considered  as  unity  in  the  case  of  orifices 
having  a  higher  coefficient  of  contraction  than  that  for  a  round  orifice.  This 
latter  coefficient,  and  consequently  the  volume  discharged,  may  be  greatly  in- 
creased by  substituting  for  the  round  orifice  pipes  or  openings  of  such  form  as 
to  render  easier  the  outflow  of  the  fluid ;  hence  the  importance  of  careful  atten- 
tion to  this  matter. 

The  coefficient  of  efflux  of  air  through  an  orifice  in  a  thin  plate  varies  some- 
what with  the  diameter  of  the  orifice  and  the  ratio  of  the  pressures.  Thus  with 
an  orifice  i  centimeter  in  diameter  Weisbach  found  the  coefficient  to  vary  from 
0.555,  wnen  the  greater  pressure  was  1.05  times  that  of  the  lesser,  to  0.788 
when  the  ratio  was  at  2.15  to  i.  With  an  opening  2.14  centimeters  in  diameter, 
the  coefficient,  with  the  ratio  of  pressures  of  1.05  to  i,  was  0.558,  and  0.723 
when  the  ratio  was  2.01  to  i. 

The  continuation  of  the  orifice  in  the  form  of  a  pipe  serves  to  increase  the 
coefficient  of  efflux,  as  is  further  evident  from  the  experiments  of  Weisbach. 
With  a  tube  i  centimeter  in  diameter  and  2  centimeters  long,  the  coefficient 
was  found  to  be  0.730  for  a  pressure  ratio  of  1.05  to  i,  and  0.830  when  the  ratio 
was  1.30  to  i.  A  short  pipe,  i  centimeter  in  diameter  and  1.6  centimeters  long, 
with  its  inlet  well  rounded  off,  thereby  rendering  easier  the  entrance  of  the  air, 
showed  a  coefficient  which  averaged  0.976  under  different  pressure  ratios  be- 
tween 1.24  and  2.14.  A  complete  nozzle  consisting  of  a  conical  tube  with  an 
angle  of  convergence  of  6°,  which  was  145  centimeters  long,  i  centimeter  in 
diameter  at  the  outlet  and  3.8  centimeters  in  diameter  at  the  inlet,  which  was 
rounded  off,  gave  for  a  ratio  of  1.08  to  i  a  coefficient  of  0.932,  and  for  a  ratio 
of  2.16  to  i  a  coefficient  of  0.984. 


1 52  MECHANICAL   DRAFT. 

In  round  numbers  the  coefficient  of  efflux,  when  the  pressure  differences  are 
comparatively  small,  as  in  the  case  of  a  fan,  may  be  taken  as  follows :  — 

For  an  orifice  in  a  thin  plate  .  .  .  .  .  0.56 
For  a  short  cylindrical  pipe  .  .  .  .  .  °-75 
For  a  rounded-off  conical  mouthpiece  ....  0.98 
For  a  conical  pipe  whose  angle  of  convergence  is  about  6°,  0.92 

Of  course  the  proper  coefficient  must  be  applied  in  any  given  case.  But  it  is 
simple  to  calculate  the  volume  of  air  which  at  a  given  velocity  would  pass  in  a 
stream  of  known  effective  area.  The  results  of  such  calculations  are  given  in 
Table  No.  91,  in  which  is  indicated  the  volume  of  air  passing  per  minute  at  the 
stated  velocity  in  a  stream  of  one  square  inch  effective  area.  How  much  larger 
than  this  area  the  orifice  should  be  in  order  to  make  the  contracted  vein  one 
square  inch  in  area  must  of  course  depend  upon  the  form  of  the  opening. 

In  most  cases  the  weight  of  air  moved  by  any  given  means  is  the  final  de- 
sideratum ;  therefore,  due  allowance  must  be  made  for  variations  in  temperature. 
The  necessity  of  this  is  shown  in  Table  No.  91,  in  columns  4  and  5,  which  in- 
dicate respectively  the  relative  weights  of  air  moved  at  a  given  velocity  and  the 
relative  velocities  necessary  to  move  the  same  weight  under  various  temperatures. 

Work  Required  to  Move  Air.  —  The  theoretical  amount  of  energy,  as  ex- 
pressed in  foot-pounds,  which  is  expended  in  moving  a  given  volume  of  air,  is 
measured  by  the  product  of  the  distance  moved  and  the  total  resistance  which 
is  overcome.  Thus,  as  per  Table  No.  91,  under  a  pressure  of  8  ounces  per 
square  inch  the  velocity  of  issuing  air  is  14,387.9  feet  per  minute.  If  the  effec- 
tive area  of  discharge  be  6  square  inches,  the  total  pressure  becomes  6  x  8  =  48 
ounces,  or  3  pounds  ;  and  the  work  done  is  14,387.9  X  3  =  43,163.7  foot-pounds. 

As  this  work  is  accomplished  in  one  minute,  it  equals  ^L'-1-  ^J~  =  1.31  H.  P. 

33,000 

In  this  manner  the  theoretical  horse-power  has  been  calculated  for  one  square 
inch  of  effective  area  under  the  temperature,  pressure  and  velocity  conditions 
of  Table  No.  91,  and  therein  incorporated  in  the  last  column.  How  much  more 
than  this  theoretical  amount  will  be  actually  required  must  depend  upon  the 
efficiency  of  the  machine  or  device  by  which  the  result  is  accomplished.  Evi- 

P 

dently,  with  a  constant  velocity  due  to  a  constant  head,  h  =  — ,  the  actual  pres- 
sure must  vary  directly  as  the  density  of  the  air  and  inversely  as  its  absolute 
temperature.  This  has  already  been  explained  and  is  presented  in  column  3  of 
Table  No.  93.  Therefore,  if  the  velocity  remains  constant,  the  power  required 
to  overcome  the  resistance  must  be  exactly  proportional  to  the  relative  pressure. 


MECHANICAL    DRAFT. 


'53 


Table  No.  93. —  Influence  of  the  Temperature  of  Air  upon  the  Conditions  of  its 

Movement. 


Temper- 
ature in 
Degrees. 
Fahr. 

Relative 
Velocity  Due 
to  the  Same 
Pressure. 

Relative 
Pressure 
Necessary  to 
Produce 
the  Same 
Velocity. 

3 

Relative 
Weight  of  Air 
Moved 
at  the  Same 
Velocity. 

4 

Relative 
Velocity 
Necessary  to 
Move  the 
Same  Weight 
of  Air. 

5 

Relative 
Pressure 
Necessary  to 
Produce 
the  Velocity  to 
Move  the 
Same  Weight 

6 

Relative 
Power  Neces- 
sary to  Move 
the  Same 
Volume  of  Air 
at  the  Same 
Velocity. 

7 

Relative 
Power   Neces- 
sary to  Move 
the  Same 
Weight  of  Air 
at  the  Velocity 
in  Column  5 
and  the 
Pressure  in 
Column  6. 
8 

300 

0.98 

1.04 

1.04 

0.96 

0.96 

1.04 

0.92 

40 

0.99 

I.  O2 

I.  O2 

0.98 

0.98 

1.02 

0.96 

5° 

1.  00 

1.  00 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

60 

I.OI 

0.98 

0.98 

1.02 

I.  O2 

0.98 

1.04 

70 

1.  02 

0.96 

0.96 

1.04 

1.04 

0.96 

1.  08 

80 

1.03 

0.94 

0-94 

1.  06 

1.  06 

0.94 

1.  12 

90 

1.04 

o-93 

093 

1.  08 

1.  08 

o-93 

I.I7 

100 

1.05 

0.91 

0.91 

1.  10 

1.  10 

0.91 

1.  21 

125 

1.07 

0.87 

0.87 

I.I5 

1-15 

0.87 

I.32 

150 

1.09 

0.84 

0.84 

1.20 

1.20 

0.84 

1-43 

175 

I.  II 

0.8  1 

0.8  1 

1.24 

1.24 

0.8  1 

«•« 

200 

•    1.14 

0.78 

0.78 

1.29 

1.29 

0.78 

1.67 

225 

1.16 

0-75 

0-75 

i-34 

i-34 

0-75 

i.  80 

250 

1.18 

0.72 

0.72 

i-39 

r-39 

0.72 

1-93' 

275 

1.20 

0.69 

0.69 

1.44 

1.44 

0.69 

2.07 

300 

1.22 

0.67 

0.67 

1.49 

1.49 

0.67 

2.22 

325 

1.24 

0.65 

0.65 

i-54 

i-54 

0.65 

2.36 

350 

1.26 

0.63 

0.63 

1.59 

i-59 

0.63 

2-51 

375 

1.28 

0.61 

0.61 

1.63 

1.63 

0.6  1 

2.66 

400 

I.30 

o-59 

o-59 

1.68 

1.68 

o-59 

2.82 

425 

1.32 

0.58 

0.58 

x-73 

J-73 

0.58 

2-99 

45° 

i-34 

0.56 

0.56 

1.78 

1.78 

0.56 

3-!7 

475 

'•35 

o-55 

o-55 

1.83 

1.83 

0.55 

3-35 

500 

'•37 

o-53 

o-53 

1.88 

1.88 

o-53 

3-53 

525 

i-39 

0.52 

0.52 

i-93 

i-93 

0.52 

3-72 

55° 

1.41 

0.51 

0.51 

1.98 

1.98 

0.51 

3-92 

575 

i-43 

0.49 

0.49 

2.03 

2.03 

0.49 

4.12 

600 

i-44 

0.48 

0.48 

2.08 

2.08 

0.48 

4-33 

625 

1.46 

0.47 

0.47 

2-J3 

2.13 

0.47 

4-54 

650 

.48 

0.46 

0.46 

2.18 

2.18 

0.46 

4-75 

675 

•49 

o-45 

0-45 

2.22 

2.22 

0-45 

4-93 

700 

•Si 

0.44 

0.44 

2.27 

2.27 

0.44 

Vs 

725 

•52 

0-43 

0.43 

2.32 

2.32 

0-43 

5-38 

750 

•54     ' 

0.42 

0.42 

2-37 

2-37 

0.42 

5.62 

775 

•56 

0.41 

0.41 

2.42 

2.42 

0.41 

5-86 

800 

•57 

0.40 

0.40 

2.47 

2-47 

0.40 

6.10 

'54 


MECHANICAL   DRAFT. 


As  a  consequence,  the  values  given  in  column  7  are  identical  with  those  in 
column  3.  The  velocity  being  constant,  the  volume  discharged  is  also  constant, 
but  its  relative  weight  is  as  shown  in  column  4. 

If  it  be  desired  to  pass  through  the  same  orifice  a  constant  weight  of  air,  its 
velocity  must  necessarily  vary  directly  with  its  increase  in  absolute  temperature, 
for  its  density  correspondingly  decreases.  The  velocity  necessary  to  move  the 
same  weight  is  produced  under  each  different  temperature  by  the  relative  pres- 
sure shown  in  column  6.  The  pressure  thus  necessary  to  produce  this  velocity 
must  at  constant  temperature  evidently  increase  with  the  square  of  the  velocity, 
and  at  other  temperatures  must  coincidently  decrease  inversely  with  the  abso- 
lute temperature ;  that  is,  proportionately  to  the  density. 

For  illustration  take  the  case  of  air  at  a  temperature  of  300°.  Per  the  table, 
column  5,  the  velocity  necessary  to  move  the  same  weight  as  at  50°  is  relatively 
1.49.  For  its  production  this  would  call  for  a  relative  pressure  of  i.4g2  =2.22 
at  50°,  but  at  the  temperature  of  300°  the  pressure  required  to  produce  the 
given  velocity  is,  per  column  3,  only  0.67  of  that  required  at  50°.  Hence  the 
relative  pressure  required  at  300°  to  produce  the  velocity  necessary  to  move 
the  same  weight  of  air  is  relatively  2.22  X  0.67  =  1.49  times  that  which  is 
necessary  to  produce  the  movement  of  the  same  weight,  but  less  volume,  at  50°. 
Under  these  conditions  of  moving  the  same  weight  at  different  temperatures, 
the  relative  power  required  is  evidently  the  product  of  the  factors  in  column  5 
and  in  column  6,  for  it  is  represented  by  the  product  of  the  pressure  into  the 
velocity.  Upon  this  basis  column  8  has  been  calculated.  From  this  is  evident 
the  fact  that  the  work  performed  is  not  proportional  to  the  weight  of  the  air 
moved,  but  to  the  distance  through  which  the  resistance  is  overcome. 

The  power  required  to  move  air  by  means  of  a  fan  is  of  great  importance  in 
the  consideration  of  any  system  of  mechanical  draft.  Other  things  equal,  and 
friction  neglected,  the  power  required  to  drive  a  fan  increases  as  the  cube  of 
its  speed.  For  the  pressure  increases  as  its  square,  the  velocity  obviously 
increases  as  its  speed,  and  the  work  done  is  the  product  of  these  two  factors. 
Furthermore,  the  speed  remaining  constant,  the  volume  also  remains  constant, 
while  the  weight  of  air  moved  and  the  power  required  both  decrease  in  propor- 
tion to  the  density  of  the  air ;  that  is,  inversely  as  its  absolute  temperature. 
The  subject  of  fans  will  be  discussed  at  length  in  the  chapter  on  Mechanical 
Draft.  The  cause  for  the  enormous  waste  of  energy  in  the  movement  of  air  by 
a  chimney  is,  as  explained  in  the  chapter  on  Chimney  Draft,  due  to  the  fact 
that  the  energy  is  not  directly  applied,  as  with  a  fan,  but  that  the  air  move- 
ment is  secured  by  the  expenditure  of  heat  in  raising  the  temperature,  and 
reducing  the  density  of  the  gases  so  that  gravity  may  act  to  produce  the  flow. 


MECHANICAL    DRAFT.  155 

Movement  of  Air  in  Pipes.  —  Air  in  its  movement  in  pipes  or  conduits  is 
resisted  by  its  friction  upon  their  interior  surfaces.  This  resistance  of  friction 
is  proportional  to  the  surface  with  which  it  comes  in  contact  ;  that  is,  directly  to 
the  length  and  inversely  to  the  diameter  of  the  pipe.  It  also  varies  as  the 
square  of  the  velocity,  and  is  expressed  by  Weisbach's1  well-known  formula  — 


In  which  f  =  coefficient  of  resistance  of  friction,  to  be  determined  by  experiment. 
/  =  length  of  pipe. 
d  =  diameter  of  pipe. 
v  =  velocity  of  the  air. 
g  =  acceleration  due  to  gravity  =32.16. 

The  value  of  /evidently  controls  the  result,  and  must  depend  both  upon  the 
material  and  character  of  construction  of  the  pipe.  Assuming  the  pipe  to  be  of 
galvanized  iron,  carefully  made  and  erected  with  all  internal  laps  extending  in 
the  direction  of  the  air  movement,  the  following  formulae,  with  constants  in 
round  numbers,  have  been  deduced  from  that  previously  given:  — 


_       lv2  .  ^ V'  25,000^ 

25,000^  / 

25,000^  ,  lv2 

V2 


In  all  of  which/  =  loss  of  pressure  in  ounces  per  square  inch. 
"•  =  velocity  in  feet  per  second. 
/  =  length  of  pipe  in  feet. 
d  =  diameter  of  pipe  in  inches. 

Taking  the  weight  of  one  cubic  foot  of  air,  in  round  numbers,  as  0.08  pounds, 
and  expressing  the  area  of  the  pipe  by  A,  the  horse-power  lost  in  friction  in  a 
pipe  100  feet  long  may  be  determined  by  the  formula  — 


By  means  of  the  formulas  for  loss  of  pressure  and  horse-power  lost  in  friction, 
Table  No.  94  has  been  calculated  for  pipes  of  various  diameters,  all  100  feet 
long,  with  air  travelling  at  different  velocities  expressed  in  feet  per  minute.  No 


i  Mechanics  of  Engineering.     Julius  Weisbach,  Ph.  D.     Translated  by  Eckley  B.  Coxe,  A.  M. 
New  York,  1878. 


MECHANICAL    DRAFT. 


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43d  }33jl  Ut 


MECHANICAL   DRAFT. 


'59 


allowance  has  been  made  for  differences  of  temperature  between  the  ends  of 
the  pipe.  For  any  other  length  of  pipe  the  losses  will  be  directly  proportional. 
From  this  table  is  evident  the  desirability,  and  in  many  cases  the  necessity,  of 
making  pipes  of  ample  area.  Suppose,  for  illustration,  it  is  desired  to  move  a 
given  volume  of  air,  such  that,  if  passed  through  a  6-inch  pipe,  the  velocity 
would  be  4,000  feet  per  minute.  The  horse-power  lost  in  friction  is  shown  by 
the  table  to  be  0.6346.  If  this  same  volume  were  to  be  passed  through  a  12- 
inch  pipe  which  has  four  times  the  area,  the  velocity  would  require  to  be  only 
cne-fourth  as  great,  or  1,000  feet  per  minute;  and  the  loss  in  horse-power,  per 
the  table,  would  be  only  0.0198,  or  one  thirty-second  of  that  expended  in  over- 
coming the  resistance  of  a  6-inch  pipe.  The  power  in  each  instance  being  only 
that  which  is  necessary  to  overcome  the  resistance,  there  is  further  required  an 
expenditure  of  power  to  actually  move  the  air.  These  two  items  of  power,  that 
necessary  to  overcome  the  pressure  head,  and  that  required  to  produce  the 
velocity  head,  must  be  added  together  to  determine  the  total  amount  of  power 
necessary  to  move  the  air  under  the  two  conditions. 

Tremendous  losses  are  evident  in  the  case  of  small  pipes,  but  they  become  of 
relatively  far  less  importance  in  the  larger  sizes.  Thus  the  horse-power  loss  in 
the  6-inch  pipe,  when  the  velocity  is  4,000  feet  per  minute,  is  0.6346  ;  and  that 
in  a  24-inch  pipe,  with  the  same  velocity,  is  2.5382.  But  the  latter  pipe  having 
1 6  times  the  area  of  the  former,  the  actual  loss  per  unit  volume  moved  in  the 

latter  is  2'^3       =  0.1586,  or  one-fourth  of    that  lost  in   the  movement  of    the 

same  volume  of  air  at  the  same  velocity  in  the  6-inch  pipe. 

The  conclusion  to  be  drawn  from  this  table  is  that  excessively  small  pipes 
and  high  velocities  should,  so  far  as  possible,  be  avoided ;  but  that,  after  a 
reasonable  size  is  reached,  or  the  velocity  is  brought  down  to  a  moderate  rate, 
a  change  in  either  size  or  velocity  will  have  only  a  relatively  small  effect  upon 
the  loss  in  pressure  and  power.  It  is,  therefore,  evident  that  if  the  attempt  to 
reduce  the  losses  is  carried  to  an  extreme,  other  factors,  such  as  the  cost  of  the 
pipe  and  the  space  occupied,  may  turn  such  apparent  saving  into  a  practical 
loss  from  a  commercial  standpoint. 

Measurement  of  Draft.  —  Draft  is  usually  measured  by  the  difference  in  level 
of  a  liquid  in  the  arms  of  a  tube  of  U  form  having  one  end  open  to  the  atmos- 
phere and  the  other  connected  with  the  enclosed  space  within  which  a  different 
pressure  exists.  The  preponderance  of  pressure  in  one  arm  forces  the  liquid 
downward  and  causes  a  corresponding  rise  in  the  other.  The  difference  in  level 
represents  the  height  of  a  column  of  the  liquid  which  will  be  sustained  by  the 
excess  of  pressure.  For  large  pressure  differences,  mercury  is  used  because  of 


i6o 


MECHANICAL    DRAFT. 


Table  No.  95.  —  Pressures  in  Ounces  per  Square  Inch  Corresponding  to  Various 
Heads  of  Water  in  Inches. 


Head  in  Inche 


Decimal  Parts  of  an  Inch. 


.0           \            .1 

•' 

•3 

•4 

•5 

.6 

•7 

.8 

•9 

0 

0.06 

0.12 

0.17 

0.23 

0.29 

o-35 

0.40 

0.46 

0.52 

I 

0.58 

0.63 

0.69 

0-75 

0.8  1 

0.87 

o-93 

0.98 

1.04 

1.09 

2 

1.16 

1.  21 

1.27 

*-33 

i-39 

1.44 

1.50 

I.S6 

1.62 

1.67 

3 

'•73 

1.79 

1.85 

1.91 

1.96 

2.02 

2.08 

2.14 

2.19 

2.25 

4 

2.31 

2-37 

2.42 

2.48 

2-54 

2.60 

2.66 

2.72 

2-77 

2.83 

5 

2.89 

2.94 

3-00 

3.06 

3.12 

3-18 

3-24 

3-29 

3-35 

3-41 

6 

3-47 

3-52 

3.58 

3-64 

3-70 

3-75 

3.81 

3-87 

3-92 

3-98 

7 

4.04 

4.10 

4.l6 

4.22 

4.28 

4-33 

4-39 

4-45 

4^5° 

4-56 

8 

4.62 

4.67 

4-73 

4-79 

4-85 

4.91 

4-97 

5-°3 

5-08 

5.14 

9 

5.20 

5-26 

5-3i 

5-37 

5-42 

5-48 

5-54 

5.60 

5.66 

5-72 

the  relatively  small  range  which,  owing  to  its  great  density,  is  required  to  show 
great  differences  in  pressure.  Water,  however,  is  ordinarily  employed  in  the 
measurement  of  draft  in  connection  with  steam  boilers.  One  cubic  foot  (1,728 
cubic  inches)  of  water  at  50°,  the  temperature  at  which  the  preceding  calcula- 


Table  No.  96. —  Height  of  Water  Column  in  Inches  Corresponding  to  Various 
Pressures  in  Ounces  per  Square  Inch. 


Pressure  in  Ounces 


Decimal  Parts  of  an  Ounce. 


per  oquare  men. 

- 

•' 

•» 

•3 

.4 

.5 

.6 

•7 

.8 

•9 

0 

0.17 

o-35 

0.52 

0.69 

0.87 

1.04 

1.  21 

I.3S 

I.56 

I 

J-73 

1.90 

2.08 

2.25 

2.42 

2.60 

2-77 

2-94 

3-" 

3-29 

2 

3-46 

3-63 

3.81 

3.98 

4-15 

433 

4-50 

4.67 

4.84 

S-oi 

3 

5.19 

5-36 

5-54 

5-71 

5.88 

6.06 

6.23 

6.40 

6-57 

6-75 

4 

6.92 

7.09 

7.27 

7-44 

7.61 

7-79 

7.96 

8.13 

8-30 

8.48 

5 

8.65 

8.82 

9.00 

9.17 

9-34 

9-52 

9.69 

9.86 

10.03 

IO.2I 

6 

10.38 

iQ-55 

10.73 

10.90 

11.07 

11.26 

"•43 

1  1.  60 

11.77 

11.95 

7 

12.  II 

12.28 

12.46 

12.63 

12.80 

12.97 

I3-I5 

J3-32 

'3-49 

I3-67 

8 

13.84 

14.01 

14.19 

14.36 

M-53 

14.71 

14.88 

1S-°S 

15.22 

1540 

9 

I5-57 

15-74 

'5-92 

1609 

16.26 

16.45 

16.62 

16.79 

16.96 

I7.I4 

MECHANICAL   DRAFT. 


161 


tions  have  been  made,  weighs  62.409  pounds;  therefore  a  column  of  water  of 
this  temperature  1,728  inches  high  and  i  square  inch  cross-sectional  area  would 
exert  a  pressure  of  62.409  pounds  per  square  inch,  and  a  pres- 
sure of  i  pound  per  square  inch  would  be  exerted  by  a  column 


— — —  =  27.7  inches  high. 
62.409 


From  this  it  is  readily  deduced  that 


an  ounce  pressure  per  square  inch  is  produced  by  a  water  column 
1.73  inches  high,  and  that  i  inch  head  of  water  is  equivalent  to 
a  pressure  of  0.578  ounces  per  square  inch.  Table  No.  95  serves 
to  show  these  relations  for  different  heights  of  water  column. 
Table  No.  96  indicates  the  height  of  water 
column  corresponding  to  any  given  pressure 
in  ounces  per  square  inch. 

The  simplest  form  of  U-tube  glass  draft  is 
shown  in  Fig.  5.      The  glass  tube  is  carefully 
fitted  and  securely  attached  to  the  wood  back. 
The  scale  is  laid  out  so  as  to  indicate  total 
differences  in  level,  for  which  purpose  it  is 
necessary  to  have  the  water  stand  at  exactly 
zero  in  both  tubes  when  they  are  open  to  the 
atmosphere.    For  reading  other 
pressures,  a  rubber   tube    may 
be  slipped  over  the  end  of  the  FIG.  6. 

left-hand  tube,  where  the  wood    WATER  GAUGE  FOR 
back  is  cut  away. 

A  somewhat  unique  and  exceedingly  compact 
form  of  gauge,  acting  upon  this  principle,  may  be 
constructed  by  making  the  glass  tubes  of  consider- 
ably different  diameters  and  inserting  one  within 
the  other.  The  lower  end  of  the  outer  tube  should 
be  permanently  closed,  and  the  inner  tube  held 
rigidly  in  position,  with  its  lower  end  just  out  of 
contact  with  the  bottom  of  the  outer  tube.  If  con- 
nection be  made  to  the  top  of  the  inner  tube,  the 
change  in  level  may  be  clearly  seen  in  the  outer 
tube,  upon  the  surface  of  which  the  graduations  may  be  placed. 

A  substantial  form  of  gauge,  manufactured  by  the  B.  F.  Sturtevant 
Co.,  and  capable  of  measuring  up  to  20  ounces  pressure,  is  illustrated 
in  Fig.  6. 


FIG.  5. 

WATER  GAUGE  FOR 
Low  PRESSURES. 


l62 


MECHANICAL    DRAFT. 


One  arm  of  the  U  tube  is  made  of  brass  tubing,  and  provided  with  a  cup  at 
the  top  of  such  size  that  practically  constant  level  is  maintained  therein  under 
the  different  conditions  of  pressure.  The  scale  is  constructed  with  this  level  as 
its  zero  mark. 

Gauges  of  the  form  and  construction  just  described  serve  all  ordinary  pur- 
poses, but  lack  the  refinement  necessary  to  the  determination  of  very  small  dif- 


ferences of  pressure, 
special  devices  are 
these  is  shown  in 
bodies  the  principle 
the  level  of  the  wa- 
ascertained  by  the 
As  is  evident  in  the 
strument  consists, 
tubes  of  relatively 
in  one  casting.  The 
near  the  bottom 
whereby  a  constant 
within  them  may  be 
as  they  are  both  ex- 
pressure.  The  hook 
that  its  point  is  just 
water,  and  the  mi- 


FIG.  7. 
HOOK   DRAFT  GAUGE. 


For  this  purpose 
necessary.  One  of 
Fig.  7.  This  em- 
of  the  U  tube;  but 
ter  is  very  closely 
aid  of  a  hook  gauge, 
illustration,  this  in- 
in  effect,  of  two 
large  diameter  made 
tubes  communicate 
through  an  opening, 
level  of  the  water 
maintained  so  long 
posed  to  the  same 
having  been  set  so 
at  the  surface  of  the 
crometer  reading 


having  been  taken,  connection  may  be  made  to  the  air  cock  at  the  left,  and  the 
hook  again  adjusted  at  the  new  level.  The  micrometer  screw,  with  its  gradu- 
ated head,  makes  the  reading  of  pressure  differences  to  —  —  of  an  inch  a  com- 
paratively simple  matter. 

Evidently,  such  a  gauge  gives  only  relative  readings,  but  may  be  so  graduated 
as  to  indicate  the  total  difference  in  water  levels.  These  two  readings,  the  dif- 
ference between  which  gives  the  pressure  in  inches  of  water,  render  it  unneces- 
sary to  bring  the  water  to  any  stated  level  under  atmospheric  pressure. 

An  extremely  delicate  but  easily  read  draft  guage  is  that  described  by  Weis- 
bach,  under  the  name  of  the  Wollaston  Anemometer,  and  perfected  by  Messrs. 
J.  C.  Hoadley  and  F.  H.  Prentiss.1  As  constructed  and  used  by  them,  it  con- 
sists of  two  glass  tubes  (Fig.  8)  about  30  inches  long  and  about  0.4  inches 
diameter  inside,  connected  at  each  end,  by  means  of  stuffing-boxes,  to  suitable 


Warm-Blast  Steam-Boiler  Furnace.     J.  C.  Hoadley.     New  York,  1886. 


MECHANICAL    DRAFT. 


163 


tubular  attachments,  through  which  they  are  secured  to  a  backing  of  wood.  A 
stop-cock  in  each  of  these  attachments  serves  to  establish  or  shut  off  communi- 
cation between  the  glass  tubes.  Connecting  with  the  top  of  each  is  a  brass 
drum  4.25  inches  in  diameter,  with  heads  of  glass.  Each 
drum  is  provided  with  a  nipple  and  stop-cock  for  connection 
by  tube  to  any  desired  space.  Two  sliding  scales  are  pro- 
vided between  the  glass  tubes,  to  measure,  one  the  depres- 
sions, and  the  other  the  elevations,  of  the  liquid  filling  the 
lower  half  of  the  tubes. 

The  lower  stop  being  open,  the  two  tubes  are  filled  up 
to  about  the  middle  of  their  heights  with  a  mixture  of 
alcohol  and  water.  The  lower  stop-cock  is  then  closed,  the 
upper  one  opened,  and  crude  olive  oil  is  carefully  poured 
in  until  it  fills  the  first  tube  up  to  the  upper  cross  tube, 
whence  it  flows  into  the  second  tube,  and  so  finally  fills 
both  tubes  and  rises  to  the  middle  of  both  drums. 

The  oil  forms,  with  the  water-and-alcohol  mixture,  a  very 
fine  meniscus,  which  is  readily  discernible  because  of  the 
contrast  in  colors.  The  specific  gravity  of  oil  should  differ 
from  that  of  the  mixture  by  at  least  i  per  cent  to  avoid  the 
tendency  to  mix.  For  most  purposes,  a  difference  of  2  per 
cent  in  the  specific  gravity  will  give  sufficient  sensitiveness 
—  fifty  times  as  much  as  a  water  column. 

The  method  of  using  this  instrument  is  as  follows  :  The 
stop-cocks  between  the  tubes  and  those  on  the  drums  are 
opened  and  the  liquids  allowed  to  come  to  a  level.  If,  now, 
one  of  the  drums  be  connected  with  a  suction  fan  or  chimney, 
the  diminished  pressure  will  cause  the  oil  to  flow  up  into 
that  drum.  The  surface  of  the  oil  in  the  drum  is  about  100 
times  as  large  as  the  inside  cross-section  of  the  glass  tubes, 
and  in  the  same  proportion  will  the  rise  of  the  lower  liquid 
on  the  one  side,  and  its  depression  on  the  other,  exceed  the 
corresponding  rise  and  depression  of  the  upper  surface  of 
the  oil. 

If,  now,  when  equilibrium  has  been  restored,  the  lower  stop-cock  be  closed, 
the  upper  one  opened,  and  the  connection  with  the  fan  or  chimney  severed,  the 
lower  liquid  will  be  kept  immovable,  while  the  oil  will  flow  through  the  upper 
cross-tube,  and  come  again  to  common  level  in  the  two  drums.  On  connecting 
again  with  the  flue  or  chimney,  and  again  closing  the  upper  stop-cock  and  open- 


FIG.  8. 

MODIFIED 

WOLLASTON 

ANEMOMETER. 


i64 


MECHANICAL    DRAFT. 


ing  the  lower  one,  a  diminished  repetition  of  the  former  action  will  take  place; 
the  lower  liquid  will  rise  a  little  in  one  tube,  and  fall  a  little  in  the  other,  and 
the  surface  level  of  the  oil  in  the  two  drums  will  become  slightly  unequal.  A 
few  repetitions  of  this  process  will  bring  the  difference  in  level  of  the  lower 
liquid  in  the  two  tubes  to  represent  the  entire  difference  in  pressure  on  the  sur- 
face of  the  oil  in  the  two  drums.  That  is,  a  certain  known  height  of  column, 
filled,  in  one  tube  with  a  mixture  of  alcohol  and  water,  with  the  fan  or  chimney 
pressure  on  its  surface,  is  just  balanced  by  an  equal  height  of  column  filled  with 
olive  oil,  with  the  pressure  of  the  atmosphere  on  its  surface.  If  the  specific 
gravity  of  the  oil  be  0.916,  and  that  of  the  mixture  of  water  and  alcohol  be 
0.926,  their  difference  is  only  i  per  cent.  As  specific  gravity  is  referred  to  water 
as  unity,  the  differential  column  existing  where  the  liquids  differ  in  specific 
gravity  by  only  i  per  cent  represents  the  same  effect  as  that  of  a  water  column 

— —  part  as  high  or  a  mercury  column  — ?—  part  as  high.  This  instrument, 
100  1,360 

with  the  respective  specific  gravities  of  0.976  and  0.937,  was  sensitive  enough, 
as  applied  by  Hoadley,  to  show  plainly  the  reduction  of  chimney  draft  caused 
by  opening  a  sliding  register  in  the  fire  door  for  the  admission  of  air  above  the 
fire,  although  the  aggregate  area  of  the  opening  was  only  six  square  inches. 

All  of  the  gauges  which  have  been  described  are  designed  only  for  indepen- 
dent observations,  so  that  an  approach  to  a  continuous  record  can  only  be  se- 
cured by  a  multitude  of  read-  ^^.-  —  -«^  ings  taken  at  very  short  inter- 


vals. The  impracticability 
the  advantages  of  an 
own  operation  records 
tensity  of  the  draft, 
cal  Draft  Recorder 
instrument,  which  is 
for  this  Company  by 
Gage  and  Valve  Corn- 
essential  parts.  First, 
which  operates  a  prac- 
ton  under  the  influ- 
The  motion  of  this 
a  steam-engine  indi- 
the  attached  arm, 


FIG.  9. 
MECHANICAL  DRAFT  RECORDER 


of  such  a  method  points  to 
instrument  which  by  its 
the  changes  in  the  in- 
Such  is  the  Mechani- 
shown  in  Fig.  9.  The 
specially  constructed 
the  Crosby  Steam 
pany,  consists  of  two 
the  small  cylinder,  in 
tically  frictionless  pis- 
ence  of  the  draft, 
piston  is  like  that  of 
cator,  multiplied  by 
which  carries  at  its 


end  a  reservoir  containing  ink.  The  second  essential  portion  is  the  dial  or 
chart,  which  is  usually  graduated  so  as  to  indicate  the  pressure  or  vacuum  in 
inches  of  water.  This  chart,  which  is  of  paper,  is  held  in  place  upon  a  circular 


MECHANICAL    DRAFT. 


plate  which  is  caused  to  revolve  by  a  system  of  clockwork.  The  point  of  the 
ink  reservoir,  being  kept  elastically  in  contact  with  the  revolving  dial,  continu- 
ously records  all  variations  in  the  draft. 

In  measuring  the  pressure  exerted  by  moving  air,  both  the  velocity  head  and 
the  pressure  head  have  to  be  taken  into  account.  To  separate  these  two  factors 
of  the  total  head,  a  form  of  Pilot's  tube  may  be  employed,  as  illustrated  in  Fig. 
10,  where  it  is  applied  in  connection  with  a  pipe,  through 
the  side  of  which  it  is  inserted.  The  tube  A  is  open  at 
the  end  and  connects  by  rubber  tubing  with  one  arm  of 
an  ordinary  U-tube  water  gauge.  The  other  tube  B  is 
closed  upon  the  end,  but  has  in  its  opposite  sides  two 
small  holes,  and  is  connected  to  the  other  arm  of  the 
gauge.  Tube  A  receives  the  full  effect  of  the  current  of 
moving  air,  and  thus  tends  to  indicate  upon  the  gauge 
the  total  head,  including  both  the  velocity  head  and  the 
pressure  head.  But  the  influence  of  the  velocity  is  prac- 
tically removed  from  B,  which,  therefore,  receives  only 
the  pressure  due  to  the  pressure  head.  As  this 
tube  is  connected  to  the  other  arm  of  the  gauge, 
the  pressure  thus  indicated  is  only  that  due  to 
the  velocity  head ;  for,  both  arms  being  subject  to  I 
the  pressure  head,  these  pressures  are  balanced. 

At  high  pressures  even  this  device  is  not  alto- 
gether reliable,  for  the  air  moving  by  the  openings 
in  tube  B  has  an  aspirating  influence  which  may          FlG>  Ia    PITOT'S  TuBE- 
tend  to  produce  a  partial  vacuum  in  this  tube.     It  is,  therefore,  necessary,  before 
making  final  measurements,  to  determine  by  independent  readings  whether  this 
is  the  case,  and  to  what  extent. 

Conditions  of  Boiler  Draft. —  In  boiler  practice  the  force  of  the  draft  must  be 
expended  in  two  ways.  First,  a  portion  of  it  is  necessary  to  overcome  the 
resistances  of  the  grate  and  the  fuel  upon  it,  of  the  combustion  chamber,  flues 
or  tubes,  and  uptake,  and  of  the  means  of  connection  to  the  source  of  draft,  be 
it  fan  or  chimney.  Within  the  chimney  or  fan  certain  other  resistances  must 
also  be  overcome.  Second,  the  draft  must  in  addition  be  sufficient  to  impart 
the  necessary  velocity  to  the  requisite  amount  of  air  for  the  direct  purposes  of 
combustion.  As  has  already  been  shown,  the  velocity  thus  produced  varies 
directly  as  the  square  root  of  the  intensity  of  the  draft,  and  consequently  the 
volume  at  constant  temperature  likewise  varies  in  the  same  ratio.  The  force 
expended  in  overcoming  the  resistances  is  directly  proportional  to  the  pressure, 


1 66  MECHANICAL    DRAFT. 

— that  is,  to  the  square  of  the  velocity, —  while  the  work  done  in  moving  the  air, 
being  the  resultant  of  a  given  pressure  exerted  through  a  given  distance,  which 
is  measured  by  the  velocity,  becomes  proportional  to  the  product  of  these 
factors ;  namely,  to  the  cube  of  the  velocity.  Therefore,  if  under  stated  condi- 
tions the  resistance  be  increased,  as  by  a  thicker  fire  or  finer  coal,  the  intensity 
of  draft  required  for  overcoming  this  resistance  must  also  be  increased.  If, 
however,  the  prior  conditions  were  such  that  the  maximum  intensity  of  draft 
attainable  was  already  devoted  to  the  requirements  of  combustion,  any  demand 
for  increased  draft  to  overcome  the  greater  resistance  could  only  be  met  by 
reducing  the  amount  of  that  portion  previously  devoted  to  the  creation  of  ve- 
locity. But  a  slight  reduction  in  the  velocity  considerably  decreases  the 
expenditure  necessary  for  overcoming  the  resistance.  The  ultimate  result,  how- 
ever, is  that  the  expenditure  for  overcoming  resistance  is  increased,  while  the 
velocity  and  consequent  volume  of  air  are  decreased.  But  both  of  these  changes 
are  proportionately  less  than  would  at  first  appear.  The  reason  is  evident  in 
the  fact  that,  whereas  the  resistances  which  are  overcome  are  directly  propor- 
tional to  the  pressure  or  draft  exerted,  the  velocity  is  decreased  only  in  propor- 
tion to  the  square  root  of  the  draft,  which  is  thus  diverted  from  its  former  service 
of  producing  velocity  to  that  of  overcoming  the  added  resistance. 

In  all  boiler  practice  the  most  important  of  the  resistances  to  be  overcome  are 
those  of  the  tubes  and  of  the  grate  with  the  fuel  upon  it,  while  the  expenditure 
for  the  production,  of  velocity  is  comparatively  small.  A  careful  study  of  these 
resistances  is,  therefore,  of  importance  in  a  thorough  consideration  of  the  prac- 
tical conditions  of  draft  production. 

Prof.  Gale1  found,  in,  the  case  of  a  stationary  boiler  furnace  of  ordinary  con- 
struction, the  following  pressures  in  pounds  per  square  foot :  — 

Required  to  produce  entrance  velocity  (3.6  feet  per  second)     .     0.013 
Required  to  overcome  resistance  of  fire  grate  .         .          .     0.9 1 

Required  to  overcome  resistance  of  combustion  chamber  and 

boiler  tubes      .         .         .          .          .          .          .          .          .1.23 

Required  to  overcome  resistance  in  horizontal  flue  .          .          .     0.06 
Required  to  produce  discharge  velocity  (11.2  feet  per  second),     0.085 

Total  effective  draft  pressure 2.298 

Back  pressure  due  to  friction  in  stack       .         .         .  .     0.19 

Total  static  pressure  produced  by  chimney       .... 


i  Theory  and  Design  of  Chimneys.      Horace  B.  Gale.      Transactions  American  Society  of 
Mechanical  Engineers,  Vol.  XI. 


MECHANICAL   DRAFT.  167 

This  total  static  pressure,  which  is  given  in  pounds  per  square  foot,  is  equiva- 
lent to  0.28  ounces  per  square  inch,  or  0.48  inches  of  water. 

It  is  evident  from  this  table  that  the  greater  part  of  the  draft  pressure  is,  as 
already  stated,  necessary  to  overcome  the  resistances  presented  by  the  fuel 
and  the  boiler  tubes.  In  fact,  Rankine  asserts  that  the  throttling  action  at  the 
grate  is  ordinarily  sufficient  to  cause  a  loss  of  head  equal  to  about  three- 
quarters  of  the  whole  draft  of  a  chimney.  As  about  75  per  cent  of  this  remain- 
ing fourth  is  necessary  to  balance  the  frictional  resistances  of  the  flues  and  the 
chimney,  there  remains  only  about  one-sixteenth  of  the  total  head  for  the  pro- 
duction of  velocity.  Prof.  H.  B.  Gale1  cites  an  instance  in  which  he  found  that 
about  60  per  cent  of  the  entire  head  was  lost  by  throttling  at  the  grate,  and  only 
about  4  per  cent  of  the  total  head  was  actually  expended  in  accelerating  the 
gases.  In  this  case  the  height  of  the  chimney  was  92  feet,  and  the  tempera- 
ture of  the  gases  609°.  If  the  whole  head  had  been  employed  in  producing 
velocity  of  the  gases,  their  velocity,  as  calculated  by  Prof.  Gale,  would  have 
been  about  78  feet  per  second.  In  reality  the  mean  observed  velocity  was  only 
1 6  feet  per  second. 

Although  the  ultimate  object  of  any  means  of  draft  production  must  neces- 
sarily be  to  create  draft  or  velocity  sufficient  to  provide  the  required  amount  of 
air  and  to  carry  off  the  gases,  yet  this  portion  of  its  work  is  almost  infinitesimal 
as  compared  with  the  demand  made  for  sufficient  pressure  to  overcome  the 
resistance  of  the  fuel  and  the  boiler.  In  other  words,  the  ability  to  create 
sufficient  pressure  difference  is  the  primary  requisite  to  burning  a  given  quantity 
of  fuel,  rather  than  the  ability  to  move  a  certain  amount  of  air.  Draft-produc- 
ing apparatus  is  not,  therefore,  to  be  based  merely  upon  the  total  number  of 
cubic  feet  to  be  moved  per  hour,  as  determined  by  multiplying  the  coal  con- 
sumption by  the  allowance  of  air  per  pound  of  coal.  If  this  were  the  case,  a 
low  chimney,  or  a  large  slow-running  fan,  would  meet  the  requirements.  In 
reality,  the  relatively  immense  resistances  of  fuel  and  boiler  demand  that  the 
chimney  or  fan  shall  first  be  designed  to  create  sufficient  intensity  of  draft  to 
overcome  these  resistances  and  to  create  the  requisite  velocity.  This  velocity 
must  be  such  that,  if  multiplied  by  the  full  area  at  which  it  is  measured,  the 
product  will  equal  the  volume  of  air  necessary  for  the  combustion  of  the  stated 
amount  of  fuel.  The  height  of  chimney  or  the  diameter  and  speed  of  fan 
necessary  to  create  the  draft  thus  shown  to  be  required  having  been  determined, 
it  is  only  necessary  to  make  the  capacity  such  as  to  accommodate  the  given 
volume  of  air. 


Transactions  American  Society  of  Mechanical  Engineers,  Vol.  XI.,  p.  777. 


1 68 


MECHANICAL   DRAFT. 


n     n 


MECHANICAL    DRAFT.  169 

Of  course  the  velocity  with  which  the  air  and  gases  pass  from  the  ashpit  to 
the  uptake  changes  greatly  as  they  progress,  owing  to  the  variations  in  area  and 
temperature.  All  of  these  changes  play  their  part  in  affecting  the  draft  required 
to  secure  the  desired  results.  The  conditions  in  practice  are  very  clearly  shown 
by  Fig.  ii,  which  represents  in  graphical  form  the  relative  areas,  temperatures, 
volumes  and  velocities,  as  determined  by  one  of  B.  F.  Sturtevant  Co.'s 
expert  engineers  in  the  case  of  one  of  the  boilers  of  the  International  Naviga- 
tion Company's  steamship  Berlin.  This  boiler  was  one  of  four  connecting  with 
the  same  stack.  It  was  provided  with  the  Ellis  &  Eaves  system  of  preheat- 
ing the  air  before  admission  to  the  ashpit,  and  the  necessary  draft  was  produced 
by  four  special  Sturtevant  steam  fans,  through  which  the  gases  were  drawn  and 
thence  discharged  into  the  funnel.  The  relative  actual  observed  temperatures 
are  indicated  in  diagram  2,  together  with  a  statement  of  the  portion  of  the 
passage  to  which  they  pertain.  The  first  diagram  graphically  represents  the 
area  existing  at  the  various  points,  and  the  third  indicates  the  corresponding 
absolute  temperatures.  Taking  the  volume  of  air  entering  the  heater  tubes 
as  unity,  the  fourth  diagram  has  been  constructed,  showing  the  relative  volumes 
to  which  the  air  would  in  each  case  be  expanded  by  the  existing  temperature ; 
while  the  fifth  diagram,  also  calculated,  indicates  the  velocities  required  to  move 
the  given  volume  through  the  given  area.  Both  of  these  diagrams  are  purely 
theoretical,  for  they  make  no  allowance  for  leakage  or  for  increased  volume  due 
to  the  accession  of  the  products  of  combustion.  Diagrams  6  and  7,  however, 
present  the  conditions  as  actually  observed,  and  when  compared  with  diagrams 
4  and  5  clearly  show  the  effect  of  combustion  and  of  the  leakage,  which,  owing 
to  existing  conditions,  was  large.  The  area  through  the  fuel  and  the  factors  de- 
pending upon  it  are  only  suggested  by  the  dash  and  dot  lines. 

Relation  of  Draft  and  Rate  of  Combustion.  —  It  has  been  the  usual  practice, 
in  the  determination  of  the  draft  in  connection  with  a  given  boiler,  to  ascer- 
tain only  the  total  draft,  as  shown  by  the  application  of  a  gauge  to  the 
uptake,  chimney  or  fan.  From  such  readings  it  is  manifestly  unfair  to  draw 
conclusions  as  to  the  amount  or  intensity  of  draft  to  secure  a  given  rate  of 
combustion.  For,  first,  the  boiler  resistances  will,  with  the  same  boiler,  remain 
practically  constant,  while  the  character  of  the  fuel  and  the  fire  may  vary 
greatly ;  and,  second,  with  the  same  conditions  as  to  fuel  and  combustion, 
a  change  in  the  type  of  boiler  may  increase  or  decrease  the  resistance  due 
thereto.  It  is,  therefore,  evident  that  for  comparison  of  combustion  rates,  the 
draft  which  should  be  determined  is  that  relating  solely  to  the  supply  of  air  to, 
and  the  overcoming  of  the  resistances  in,  the  fire.  This  draft  is  obviously  the 
difference  between  the  over-  and  under-grate  pressures. 


170 


MECHANICAL    DRAFT. 


Although  it  has  already  been  shown  that  the  efficiency  of  combustion  in- 
creases and  the  required  air  supply  decreases  as  the  combustion  rate  rises, 
for 'the  sake  of  simplicity  in  the  matter  of  comparison  the  required  air  supply 
per  pound  of  coal  may  here  be  taken  as  constant  for  all  rates  of  combustion. 
The  volume  of  air  (for  equal  temperature)  then  becomes  an  index  of  its  velocity, 
and  varies  as  the  square  root  of  the  effective  pressure  or  draft.  Conversely, 
the  required  draft  will  vary  as  the  square  of  the  rate  of  combustion.  Of  course, 
for  the  purpose  of  properly  proportioning  draft-producing  apparatus,  the  resis- 
tances of  all  parts  of  the  boiler,  including  the  fuel  upon  the  grate,  should  be 
ascertained  for  all  ordinary  coals,  types  of  boiler  and  conditions  of  draft. 

The  difficulties  in  the  way  of  obtaining  such  knowledge  are  evident.  It  is 
not  easy  to  secure  identical  conditions  of  boiler  and  draft  when  testing  different 
coals,  and  even  with  the  same  coal  there  may  be  variations  in  its  size,  in  the 
manner  in  which  it  is  fired  and  in  which  the  grates  are  kept  clear  of  ashes, 
which  very  seriously  affect  the  results.  As  indicative  of  the  variation  in  draft 
pressure  required  for  different  kinds  of  coal,  rates  and  stages  of  combustion, 
the  results  in  Table  No.  97  are  presented.  These  are  from  a  test1  of  a  Coxe 
stoker  applied  to  Babcock  &  Wilcox  boilers.  This  stoker,  which  is  of  the 
travelling  chain-grate  type,  with  the  fire  upon  its  upper  surface,  is  provided 
with  four  blast  compartments  under  the  fire.  To  each  of  these  air  is  admitted 
in  the  desired  proportion  from  a  supply  furnished  by  a  Sturtevant  fan. 

Table  No.  97.  —  Draft  Conditions  with  Coxe  Stoker. 


SIZE  OF  COAL. 


Buckwheat. 

Buckwheat. 

Rice. 

Pressure  in  igniting  compartment,                inches  of  water 

0.14 

0.25 

0.44 

Pressure  in  burning  compartment,                    "               "                0.31                 0.56 

0.89 

Pressure  in  burning-down  compartment,          "               "                0.24                O-49 

0-73 

Pressure  in  burning-out  compartment,             "               " 

0.17 

0.42 

0.67 

Pressure  of  blast  of  air,  average,                     "               " 

0.24 

0-43 

0.68 

Vacuum  in  furnace,                                              "               " 

0.10 

O.I5 

0.24 

Total  furnace  draft,                                             "               " 

0.34 

0.58 

0.92 

Vacuum  in  stack  flue,                                          "               " 

0.13 

0.40 

0.58 

Total  draft, 

0.37 

0.83 

1.26 

Pounds  of  dry  coal  per  hour  per  square  foot  of  grate 

19.8 

32-9 

28.0 

i  Experiments  with    Automatic    Mechanical   Stokers.      J.  M.   Whitham.      Transactions   of 
American  Society  of  Mechanical  Engineers,  Vol.  XVII. 


MECHANICAL   DRAFT. 


171 


Mr.  Whitham,  in  the  same  paper,  also  presented  a  table  given  here  as  Table 
No.  98,  showing  the  relation  of  size  of  coal  to  results  obtained  with  a  Wilkinson 
stoker,  from  which  the  increased  draft  required  by  small  sizes  of  coal  is  made 
evident.  It  is  here  that  mechanical  draft  becomes  most  beneficial  in  making 
possible  the  combustion  of  low-grade  (because  finely  divided)  fuel. 

Table  No.  98.  —  Relative  Rates  of  Combustion  of  Small  Sizes  of  Anthracite  Coal. 


Grade  of  Coal. 

SIZE  OF  COAL  (ROUND  HOLES,  PUNCHED  PLATES). 

Relative  Rates  of 
Combustion  for 
Same  Draft. 

Pea. 

Through  ^j$  inch  and  over  9-16  inch    .         .         .         .         . 

IOO 
8c 

Rice 

Present  methods  of  boiler  testing  lack  the  refinements  in  ascertaining  the 
draft  pressure  at  different  points  which  are  necessary  to  an  intelligent  compari- 
son of  results.  This  fact  is  emphasized  by  the  results  given  in  Table  No.  99,' 

Table  No.  99.  —  Relation  of  Air  Pressure  and  Indicated  Horse-Power  per  Square 
Foot  of  Grate. 


NAME  OF  SHIP. 

Mean  Air  Pressure 
in  Fire  Rooms. 
Inches. 

Mean  I.  H.  P.  per 
Square  Foot  of  Grate 
Surface. 

Orlando    .                  

1.02 

16.17 

Undaunted       ........ 

1.87 

16.17 

Australia           .         

i-77 

17-75 

Galatea    .         .         .... 

1.14 

18.50 

Narcissus          

1.02 

16.17 

Immortalite      

2.01 

J7-93 

which  presents  the  observed  conditions  of  draft  and  mean  indicated  horse- 
power per  square  foot  of  grate,  in  the  trial  tests  of  a  number  of  steam  vessels 
almost  identical  in  their  steam-power  equipment.  Draft  was  in  each  case  pro- 
duced by  fans  discharging  into  a  closed  fire  room.  The  fact  that  under  similar 
conditions  the  fire-room  pressures  varied  from  1.02  to  2.01  inches  is,  at  least, 
sufficient  to  emphasize  the  necessity  of  readings  which  indicate  the  effective 
pressure  differences  between  ashpit  and  furnace  chamber. 


'  Artificial  Draft  and  its  Effects  on  Boiler  Construction.     E.  Lechner.     Translated  by  Asst. 
Engr.  Emil  Theiss,  U.  S.  Navy.     Journal  of  American  Society  of  Naval  Engineers,  Aug.,  1891. 


172 


MECHANICAL   DRAFT. 


Under  practically  identical  conditions  of  boiler,  coal  and  firing,  there  is  still 
opportunity,  particularly  with  mechanical  draft,  for  divergence  from  any  estab- 
lished relation,  owing  to  a  probable  decrease  in  air  volume  per  pound  of  coal  as 
the  rate  of  combustion  increases,  and  to  a  coincident  increase  in  the  thickness 
of  the  fire.  These  tend,  however,  to  counteract  each  other.  A  greater  depth 
of  fuel  on  the  grates  naturally  indicates  a  proportionately  greater  resistance. 
But  the  rate  of  combustion  always  increases  at  a  more  rapid  rate  than  the  thick- 
ness of  the  fire  ;  therefore,  the  pressure  under  which  the  greater  volume  of  air 
would  be  supplied  would  also  increase  more  rapidly  than  the  thickness  of  the 
fire,  and  hence  more  readily  tend  to  overcome  this  resistance. 

The  combined  effect  of  all  the  factors  appears  to  be  to  bring  the  rate  of  com- 
bustion to  substantially  the  theoretical  basis  first  presented  ;  viz.,  proportional 
to  the  square  root  of  the  effective  pressure.  Thus,  for  instance,  two  similar 
tests  upon  the  same  boiler  in  the  works  of  the  B.  F.  Sturtevant  Co.  gave  results 
presented  in  Table  No.  100.  The  square  roots  of  the  effective  pressures  in  the 
two  cases  are,  respectively,  0.705  and  0.542,  which  are  in  the  ratio  of  i  to  0.769, 
while  the  rates  of  combustion  are  in  the  ratio  of  i  to  0.767. 

Table  No.   100.  —  Relation  of  Draft  and  Rate  of  Combustion  in  Boiler  at 
B.  F.  Sturtevant  Co.'s. 


Designation  of  Test. 

Vacuum  in  Furnace. 

Vacuum  in  Ashpit. 

Effective  Pressure  to 
Produce  Combustion. 

Coal  Burned  per  Hour 
per  Sq.  Ft.  of  Grate. 

Inches. 

Inches. 

Inches. 

Pounds. 

A 

0.640 

0.144 

0.496 

21.45 

B 

0.372 

0.078 

0.294 

16.45 

The  results  of  a  series  of  tests  of  the  locomotive  boiler  of  a  torpedo  boat,  as 
presented  by  E.  Lechner '  in  a  discussion  of  this  subject,  are  given  in  Table  No. 
101.  Tests  numbered  i,  2,  3  and  4  were  conducted  upon  the  regular  grate 
under  artificial  draft  produced  by  a  centrifugal  fan  in  a  closed  fire  room,  while  in 
tests  numbered  5,  6  and  7  the  grate  was  reduced  to  one-half  the  area.  In  the 
first  series  of  tests  the  air  supply  per  pound  of  coal  was  reasonable  in  quantity, 
but  practically  constant  for  different  rates  of  combustion  ;  while  in  the  second 
series,  owing  to  the  existing  circumstances,  it  was  large  in  volume  but  decreased 
as  the  rate  of  combustion  increased. 


i  Artificial  Draft  and  its  Effects  on  Boiler  Construction.  E.  Lechner.  Translated  by  Assist- 
ant Engineer  Emil  Theiss,  U.  S.  Navy.  Journal  of  American  Society  of  Naval  Engineers, 
August,  1891. 


MECHANICAL   DRAFT. 


Table  No.  101.  —  Results  of  Tests  of  Locomotive  Boiler  on  Torpedo  Boat  with 
Different  Rates  of  Combustion. 


Number 
of  Test. 

Grate  Surface. 
Square  Feet. 

Heating 
Surface. 

Square  Feet. 

Ratio  of 
Heating  to 
Grate  Surface. 

Gauge 
Pressure. 

Pounds  per 
Square  Inch. 

Air  Pressure  in  Inches  of  Water. 

Fire  Room. 

Furnace. 

Chimney. 

I 

20.44 

882.32 

43-  ' 

113.6 

1.97                    1.57 

0-39 

2 









2.95 

2-36 

0-59 

3 









3-94 

3-i5 

0.91 

4 









5.90 

4-53 

1.18 

5 

10.22 

882.32 

86.2 

II3.6 

2-95                    2-1? 

o-39 

6               







5-9°               3-94 

1.  00 

7 









6.80               4.53 

1.18 

Number 
of  Test. 

Coal  per 
Hour  per 
Square  Foot 
of  Grate. 

Water 
Evaporated 
per  Hour 
from  86°. 

Water 
Evaporated 
per  Pound  of 
Coal. 

Temperature 
of  Chimney 
Gases. 

Air  Supplied 
per  Minute. 

Air  per 
Pound  of  Coal. 

Mean 
Thickness  of 
Fuel  on  Grates 

Pounds. 

Pounds. 

Pounds. 

Degrees. 

Cubic  Feet. 

Cubic  Feet. 

Inches. 

I 

53-6 

437-8 

8.16 

5180 

4,112 

225.2 

1  2.6o 

2 

63-4 

5'7-5 

8.17 

608 

5>365 

248.4 

13.40 

3 

76.4 

583-6 

7.64 

626 

6,287 

241.6 

14.17 

4 

93-4 

649-8 

7.00 

716 

7,092 

222.9 

15-35 

5 

66.7 

528.6 

7-93 

554 

5,221 

459.6 

9.84 

6 

101.4 

710.0 

7.00 

608 

6,831 

395-5 

11.82 

7 

113.0 

770.2 

6.80 

698 

7,092 

368.4 

I3-78 

The  relation  existing  between  the  rate  of  combustion  and  the  square  root  of 
the  corresponding  pressure  difference  is  presented  in  Table  No.  102.    A  compar- 

Table  No.   102.  —  Relation  of  Square  Root  of  Pressure  Difference  to  Rate  of 
Combustion. 


Number 
of  Test. 

Difference  of 
Pressure 
between  Fire 
Room  and  Fur- 

Square Root  of 
Difference  of 
Pressure 
between  Fire 

Ratio  of   Square 
Roots  of 
Difference  of 
Pressure, 

Ratio  of   Square 
Roots  of 
Difference  of 
Pressure, 

Ratio  of 
Corresponding 
Rate  of 
Combustion, 

Ratio  of 
Corresponding 
Rate  of 
Combustion, 

nace. 

Room  and 

referred  to  Test  i  referred  to  Test 

referred  to  Test 

referred  to  Test 

Inches. 

Furnace. 

No.  i  as  Unity. 

No.  5  as  Unity. 

No.  i  as  Unity. 

No.  5  as  Unity. 

' 

0.40 

0.632 

1.  00 

I.OO 

2 

o-59 

0.768 

1.  21 

1.18 

3 

o-79 

0.888 

I.40 

1.42 

4 

'•37 

I.I70 

1.85 

1-74 

5 

0.78 

0.883 

1.  00 

I.OO 

6 

1.96 

1-400 

i-59 

1.52 

7 

2.27                 1.507 

I.7I 

1.70 

1 

1 

1  74  MECHANICAL   DRAFT. 

ison  of  columns  4  and  5,  or  of  columns  6  and  7,  indicates  that  the  rate  of  com- 
bustion advances  in  approximately  the  same  proportion  as  the  square  root  of  the 
pressure  difference,  taking  each  series  of  tests  by  itself.  An  attempt  to  com- 
pare the  second  series  with  the  first  results  in  a  series  of  values  which  do  not 
continue  the  ratio  of  the  first  series,  but  which,  if  multiplied  by  a  constant 
quantity,  may  be  brought  into  accord  therewith.  The  approximate  relation 
between  rate  and  pressure  is  thus  indicated,  as  well  as  the  fact  that  a  complete 
formula,  which  shall  enable  one  to  determine  the  pressure  difference  required 
for  a  given  rate  of  combustion  for  any  kind  of  fuel,  must  include  a  constant 
which  shall  apply  only  under  the  given  conditions,  and  whose  various  values 
for  all  conditions  can  only  be  determined  by  experiment. 
Such  a  formula  would  naturally  take  the  form  —  - 


In  which  w  =  pounds  of  coal  burned. 
pl  =  pressure  in  ashpit. 
/2  =  pressure  in  furnace  chamber. 

c    =  constant   dependent  on    type    of   boiler,  kind    of  fuel,. 
depth  of  fire,  etc. 

But  until  the  value  of  c  is  determined  for  all  conditions,  the  formula  must  be 
limited  in  its  application.  In  a  word,  the  relation  between  the  effective  pressures 
and  different  rates  of  combustion  under  the  same  conditions  is  substantially 
established.  But  the  exact  draft  or  pressure  difference  required  to  maintain  a 
given  rate  of  combustion  of  a  specific  kind  of  coal  in  a  particular  type  of  boiler 
can  only  be  approximated  in  the  present  state  of  knowledge. 

The  relation  between  the  total  draft  and  the  rate  of  combustion,  which  is  that 
usually  indicated  in  the  results  of  an  ordinary  test,  is  practically  all  that  is  at 
present  available.  But  this  relation  is  not  of  necessity  the  same  as  that 
between  the  effective  pressure  and  the  rate.  In  fact,  the  constant  character  of 
the  resistances  of  the  boiler  proper  and  the  variableness  in  those  of  the  fuel 
with  different  rates  cause  this  relation  to  depart  somewhat  from  that  holding  in 
the  case  of  the  effective  pressures. 

This  is  clearly  indicated  in  the  results  of  careful  tests  of  Mr.  J.  M.  Whitham,1 
in  which  the  draft  was  taken  at  various  points  in  the  passage  of  the  air  and 
gases  through  the  furnace  and  boiler  when  coal  was  being  burned  at  different 
rates  of  combustion.  The  boiler  was  of  the  horizontal  return  tubular  type,  60 


•  The  Effect  of  Retarders  in  Fire  Tubes  of  Steam  Boilers.     J.  M.  Whitham.     Transactions 
American  Society  of  Mechanical  Engineers,  Vol.  XVII. 


MECHANICAL    DRAFT. 


inches  diameter  by  20  feet  long,  with  44  4-inch  tubes.  The  grate  was  stationary, 
5  feet  by  5  feet  4  inches,  with  46  per  cent  air  space,  the  ratio  of  heating  surface 
to  grate  surface  being  42.6.  In  certain  tests  the  tubes  were  fitted  with  retarders 
made  of  strips  of  No.  10  iron  20  feet  long,  with  a  pitch  of  ten  feet.  Cambria 
Company  coal  (run-of-mine,  bituminous)  was  used.  The  recorded  drafts  and 
rates  of  combustion  are  given  in  Table  No.  103.  The  results  are  also  graphi- 
cally presented  in  Fig.  12,  in  which  "fair"  lines  are  drawn  through  the  various 
points  plotted.  There  is  also  added  a  line  based  upon  the  relation  already 

Table  No.  103.  —  Relation  of  Draft  and  Rate  of  Combustion. 


Resistance,  in  Inches  of  Water.  Due  to                    Total  Draft  in  Inches  of  Water. 

Pounds  of 

Dry  Coal 

burned  per 

Furnace 

Pass  under 

Hour  per 
Square 
Foot  of 
Grate. 

Draft. 

Boiler  and 
through 
Tubes. 
No 

Retarders  in 
Tubes. 

Pass  over 
top  of 
Boiler. 

No 
Retarders. 
No  Return 
Pass.' 

With 
Retarders. 
No  Return 
Pass. 

Top  Pass. 
No 
Retarders. 

With 
Top  Pass 
and 
Retarders. 

Retarders. 

5 

0.04 

0.04             o.oo 

0.04 

0.08 

0.08 

0.12               0.12 

8 

O.I  I 

0.05                   O.O2 

0.04             0.16 

0.18 

0.20 

0.22 

10 

O.I3 

O.O7           •         O.O3 

0.05                   0.20 

0.23 

0.25 

0.28 

12 

0.17 

0.07           i         O.O4 

0.05 

0.24 

0.28 

0.29 

0-33 

1 

14            0.19             o.io             0.03 

0,05              0.29 

0.32         0.34 

0-37 

15                   0.20 

o.  1  1             0.03 

0.05            0.31 

0.34 

0.36 

°-39 

l6                  0.21                   O.I2                   0.03 

0.05        0.33 

0.36 

0.38 

0.41 

18             0.23             0.13             0.06 

0.05            0.36 

0.42 

0.42 

0.48 

20       i      0.24             o.i  6 

O.oS 

0.06            0.40 

0.48            0.46 

0.54 

22 

0.26 

0.18 

0.12 

0.06            0.44 

0.56 

0.50 

0.62 

25                  0.27 

0.22 

0.19 

0.06            0.49 

0.68 

0-55 

0.74 

28                   0.29 

0.24 

0.27 

0-07          0-53' 

0.80 

0.6o 

0.87 

3°            °-3° 

0.27 

0.3I 

0.07 

o-57 

0.88 

0.64 

0.95 

34 

0.32 

0.31 

0.38 

0.08 

0.63 

I.OI 

0.71                1.09 

36 

o-33 

0-34 

0.40 

0.08            0.67 

1.07 

0-75 

1.15 

40 

0.36 

0.38 

0.46 

0.08 

0.74 

i.  20 

0.82                1.28 

stated, —  that  the  rate  of  combustion  should  vary  as  the  square  root  of  the  effec- 
tive draft.  Taking  the  furnace  draft  as  representative  of  this  effective  draft, 
and  starting  at  a  rate  of  12  pounds  as  unity,  the  corresponding  theoretical 
drafts  have  been  calculated  and  plotted  for  other  rates.  These  correspond  with 
those  for  the  furnace  draft,  except  at  very  low  rates  of  combustion,  but  diverge 
decidedly  from  the  lines  representative  of  other  elements  of  the  total  draft. 


i76 


MECHANICAL   DRAFT. 


4.  8  u  6  20  24    -          z8  32  3t> 

Pounds  of  dry  coal  burned  per  hour  per  square  foot  of  grate. 

FIG.  12.     RELATION  OF  DRAFT  AND  RATE  OF  COMBUSTION. 

The  total  draft  required  for  the  efficient  combustion  of  various  kinds  of  fuels, 
as  given  by  Hutton,1  is  presented  in  Table  No.  104.  Of  course  this  must  be 
considered  as  applying  only  under  ordinary  conditions,  and  as  approximate  at 

Table  No.  104.  —  Total  Draft  Required  for  Efficient  Combustion  of  Different 
Kinds  of  Fuel. 


KIND  OF  FUEL. 

Total  Draft  in 
Inches  of  Water. 

KIND  OF  FUEL. 

Total  Draft  in 
Inches  of  Water. 

Straw    

0.20 

Slack,  very  small  . 

0.7  to  i.i 

Wood  .         .         .         .         .                   0.30 

Coal  Dust      .         .         .         . 

0.8  to  i.i 

Sawdust        ....                  0.35 

Semi-anthracite  coal 

0.9  to  1.2 

Peat,  light                                ,     . 

0.40    • 

Mixture  of  Breeze  and  Slack, 

i.o  to  1.3 

Peat,  heavy  .         .                  •.    •                0.50 

Anthracite,  round 

1.2  tO   1-4 

Sawdust  mixed  with  small  coal,    1          0.60 

Mixture  of  Breeze  and  Coal  Dust, 

1.2  to  1.5 

Steam  coal,  round 

0.4  to  0.7 

Anthracite  Slack  . 

1.3  to  1.8 

Slack,  ordinary      .          .          .               0.6  to  0.9 

i  Steam-Boiler  Construction.     Walter  S.  Hutton.     London,  1891. 


MECHANICAL   DRAFT. 


177 


the  best.  Hutton  also  gives  tables  of  draft  pressures  and  rates  of  combustion 
for  different  chimney  heights,  from  which  Table  No.  105  has  been  compiled. 
He  does  not,  however,  specify  the  exact  conditions  for  which  they  are  calculated, 
but  the  figures  are  approximately  true  for  medium-size  anthracite  coal  and 
stationary  boilers. 

Table  No.   105.  —  Rate  of  Combustion  for  Different  Total  Draft  Pressures. 


Height  of 
Chimney  above 
Grate,  in  Feet. 

Total  Draft        1  Rate  of  Combus- 

Height  of 
Chimney  above 
Grate,  in  Feet. 

Total  Draft 
Pressure  in  Inches 
of  Water. 

Rate  of  Combus- 
tion per  Hour 
per  Square  Foot  of 
Grate,  in  Pounds. 

25 

O.l82 

10                         130 

0.948 

30 

50 

0.364 

1  6                       140                    1.029 

34 

60 

0-437 

17                        150                    1.095 

40 

'      70 

0.512 

18                         180 

I-3'3                          50 

80 

0.583 

19                        200                     !-459                      6° 

90 

0.657    • 

20'                        225                     1.641                       70 

100 

0.729 

22                                     250                               1.825                                 8o 

no 

0.802 

24 

300 

2.189                      9° 

120                              0-875 

-1 

400 

2-553 

112 

Leakage  of  Air.  —  A  fruitful  source  of  poor  draft  and  decreased  efficiency 
lies  in  the  leakage  of  air  through  boiler  settings.  The  extent  of  such  infiltration 
is  frequently  surprising,  being  often  so  great  that  the  flame  of  a  match  is  drawn 
to  and  into  the  interstices  of  an  8-inch  brick  wall,  not  alone  at  fine  visible 
cracks,  but  at  mortar  joints  apparently  sound.  Evidently,  the  result  of  such 
leakage  with  suction  draft  is  to  increase  the  volume  of  air  to  be  handled  and 
to  decrease  the  temperature  ;  thereby  inevitably  reducing  the  draft  in  the  case 
of  a  chimney,  but  in  the  case  of  a  fan  of  proper  size  fortunately  tending  to 
increase  the  suction,  unless  the  volume  be  in  excess  of  the  capacity  of  the  fan. 
This  results  because,  with  a  fan  at  constant  speed,  the  intensity  of  the  draft 
increases  as  the  temperature  of  the  gases  passing  through  it  is  reduced.  Coin- 
cident with  the  reduction  of  temperature  is  an  increase  in  the  weight  of  the 
gases  —  and  hence  of  the  admitted  air  —  handled  by  the  fan  without  change  in 
speed.  With  forced  draft  beneath  the  grates  the  opposite  tendency  is  notice- 
able, and  a  more  or  less  direct  loss  of  heat  is  the  result.  Under  any  condition 
the  leakage  naturally  increases  with  the  draft  pressure,  no  matter  how  produced, 
and  under  equal  pressures  is  obviously  no  greater  with  mechanical  than  with 
chimney  draft. 

An  indication  of  the  frequent  amount  of  such  leakage  is  shown  in  the  results 
of  the  chemical  analyses  of  samples  of  gas  taken  respectively  from  the  back  end 


i78 


MECHANICAL   DRAFT. 


of  a  boiler  just  before  entering  the  tubes  and  from  the  uptake  flue,  as  presented 
in  Table  No.  1 06.  These  results  are  sufficient  to  show  the  important  necessity 
of  preventing  such  loss. 

Table  No.  106.  —  Air  Leakage  through  Boiler  Settings. 


CONDITIONS. 

Coal  consumed  per 
Square  Foot  of  Grate 
per  Hour. 
Pounds. 

Percentage  of 
Air  which  leaked  in 
between  Back  End 
and  Uptake  Flue. 

Middle  boiler  in  operation,  dampers  on  other  two  boilers  | 
closed  and  packed,                                                                 j 
Conditions  the  same   '      .. 
Three  boilers  in  operation            

21-45 
'16.45 
15.09 

22.08 

27-59 
15.40 

The  plant  consisted  of  three  boilers  in  one  battery.  In  the  first  two  tests  an 
inward  movement  of  air  was  perceptible  at  the  doors  of  the  outside  boilers  not 
in  use,  although  the  uptake  dampers  had  been  very  carefully  packed.  The 
decreased  leakage  in  the  third  case  is  principally  due  to  the  reduction  of  the 
ratio  between  exposed  surface  of  the  setting  and  the  volume  of  air  passing 
through  the  uptake  flue.  The  original  introduction  of  sheet-metal  stops  in  the 
setting  of  such  boilers  is  a  comparatively  simple  matter,  and  if  carefully  carried 
out  practically  prevents  all  leakage.  The  lack  of  such  arrangements,  however, 
tends,  in  many  instances,  to  render  misleading  the  results  of  tests  in  which 
allowance  has  not  been  made  for  the  effects  of  the  incidental  leakage. 


CHAPTER  IX. 
CHIMNEY  DRAFT. 

Principles  of  Chimney  Draft.  —  If  two  chimneys  of  identical  construction  and 
dimensions  be  connected  at  the  bottom  by  a  passage  of  the  same  cross-sectional 
area,  and  one  of  them  be  provided  at  its  base  with  a  means  of  heating  the  air, 
a  definite  air  movement  will  result  as  soon  as  heat  is  applied.  The  cold  air  in 
one  chimney,  being  heavier  than  the  heated  air  in  the  other,  will  constantly  seek 
to  secure  equilibrium  of  weights  and  pressures  by  flowing  downward  to  the  base 
of  the  heated  chimney.  As  a  natural  consequence,  the  heated  air  will  be 
forced  upward  and  the  cold  air  which  takes  its  place  will,  in  turn,  be  heated  and 
follow  the  same  course.  A  continuous  flow  will  thus  be  maintained,  its  velocity 
and  consequent  volume  being  dependent  upon  the  difference  in  density  of  the 
two  columns  of  air  ;  that  is,  upon  the  pressure  difference.  Although  the  differ- 
ence in  density  results  from  the  application  of  heat,  the  air  movement  is  purely 
mechanical  in  its  character,  and  depends  directly  upon  the  action  of  gravity. 

The  total  difference  in  pressure  upon  the  internal  bases  of  the  two  chimneys 
is  exactly  equal  to  the  difference  in  weight  of  the  two  columns  of  air  within 
them.  The  relative  difference  may  be  expressed  in  any  convenient  terms,  as 
pounds  per  square  foot,  ounces  per  square  inch,  or  by  the  height  of  a  column  of 
water,  mercury  or  other  fluid  necessary  to  balance  this  pressure.  If  a  simple 
U-tube  pressure  gauge  be  partly  filled  with  water,  one  end  connected  to  the 
base  of  the  cold  chimney,  and  the  other  to  the  base  of  the  hot  chimney,  the 
preponderance  of  weight  of  the  air  in  the  former  will  force  the  water  downward 
in  that  arm  of  the  tube  and  cause  a  corresponding  rise  in  level  in  the  other 
arm.  The  total  difference  in  level  may  be  read  in  inches  of  water  and  then 
readily  resolved  into  ounces  per  square  inch,  or  pounds  per  square  foot. 

Evidently,  there  being  no  difference  between  the  character  of  the  air  in  the 
cold  chimney  and  that  in  the  surrounding  atmosphere,  the  same  relative  pressures 
will  exist,  and  the  same  flow  will  continue  if  the  cold  chimney  be  removed  and 
the  air  be  allowed  to  directly  enter  the  base  of  the  hot  chimney.  Furthermore, 
the  relative  differences  in  the  density  and  pressure  created,  being  measured 
respectively  by  unit  volume  and  unit  area,  are  independent  of  the  cross-sectional 
area  of  the  hot  chimney.  In  other  words,  the  pressure  difference  is  dependent 


i8o 


MECHANICAL    DRAFT. 


only  upon  the  height  of  the  chimney  and  the  difference  in  density  between  the 
heated  air  within  and  the  cold  air  without.  The  terms  "hot"  and  "cold"  are,  of 
course,  only  relative,  for  the  draft  is  primarily  dependent  upon  the  actual  tem- 
perature difference. 

That  changes  in  the  temperature,  either  of  the  external  atmosphere  or  the 
gases  within  the  chimney,  have  a  most  marked  influence  upon  the  draft  is  very 
clearly  shown  by  Table  No.  107,  in  which  the  draft,  as  indicated  in  inches  of 
water,  is  given  for  a  chimney  100  feet  high,  with  various  internal  and  external 
temperatures.  For  any  other  height  of  chimney  than  100  feet  the  height  of 
the  water  column  is  directly  proportional  to  that  of  the  chimney.  Hence 
doubling  the  height  doubles  the  draft.  This  is  not  to  be  confused  with  the  fact 
that  the  velocity  which  the  draft  has  the  power  to  create  and  the  corresponding 
volume  of  air  moved  vary  as  the  square  root  of  the  height.  This  table  clearly 
indicates  the  necessity  of  high  chimney  temperatures  for  ample  draft,  and 
readily  accounts  for  the  stronger  draft  which  exists  in  cold  weather  because  of 
the  greater  temperature  difference. 

Table  No.   107.  —  Height  of  Water  Column  Due  to  Unbalanced  Pressures  in  Chimney 

loo  Feet  High. 


Temperature 
in 
Chimney. 

Temperature  of  External  Air. 

0° 

I0o 

20° 

30° 

40° 

50° 

60° 

7o° 

80° 

90° 

100° 

2000 

•453 

.419 

.384 

•353 

.321 

.292 

.263 

•234 

.209 

.182 

•157 

220 

.488 

•453 

.419 

.388 

•355 

.326 

.298 

.269 

.244 

.217 

.192 

240 

.520 

.488 

•451 

.421 

.388 

•359 

•33° 

.301 

.276 

.250 

.225 

260 

•555 

•528 

.484 

•453 

.420 

•392 

•363 

•334 

•309 

.282 

•257 

280 

•584 

•549 

•5J5 

.482 

•451 

.422 

•394 

•365 

•340 

•313 

.288 

300 

.611 

•576 

•541 

•5" 

.478 

•449 

.420 

•392 

•367 

•340 

•3*5 

320 

•637 

•603 

-568 

•538 

.505 

.476 

•447 

.419 

•394 

•367 

•342 

340 

.662 

•638 

•593 

•563 

•53° 

.501 

.472 

•443 

.419 

•392 

•367 

360 

.687 

•653 

.618 

.588 

•555 

.526 

•497 

.468 

•444 

.417 

•392 

380 

.710 

.676' 

.641 

.611 

•578 

•549 

.520 

.492 

.467 

.440 

.415 

400 

•732 

.697 

.662 

.632 

•598 

•570 

•541 

•5'3 

.488 

.461 

•436 

420 

•753 

.718 

.684 

•653 

.620 

.591 

•563 

•534 

•5°9 

.482 

•457 

440 

•774 

•739 

•705 

•674 

.641 

.612 

•584 

•555 

•53° 

•5°3 

•478 

460 

•793 

.758 

.724 

.694 

.660 

.632 

•603 

•574 

•549 

•522 

•497 

480 

.810 

.776 

.741 

.710 

.678 

.649 

.620 

.591 

.566 

•540 

•s»s 

500 

.829 

.791 

.760 

•73° 

.697 

.669 

•639 

.610 

.586 

•559 

•534 

MECHANICAL   DRAFT.  181 

For  a  full  comprehension  and  application  of  the  principles  of  chimney  draft 
it  is  necessary  to  consider  them  mathematically.  If  h  be  the  height  of  the 
chimney,  d  the  density  of  the  external  air,  and  dl  that  of  the  heated  air,  the 
pressure  difference/  for  unit  area  may  be  expressed  as  — 

p  =  hd  —  hd,  =  h  (d  —  d^ 

The  height  of  a  column  of  external  air  which  would  produce  this  pressure, 
acting  simply  by  its  weight,  may  be  found  by  dividing  the  pressure  by  the 
density  of  the  external  air.  Therefore,  if  H  represents  the  height  of  such  a 
column,  the  expression  will  be  — 


The  theoretical  velocity  with  which  the  external  air  would  enter  the  chimney, 
if  no  resistance  existed,  may  be  expressed  by  the  equation  — 


In  which  v  =  velocity,  in  feet  per  second. 

g  =  the  acceleration  due  to  gravity  =  32.16. 
H  =  head,  or  distance  fallen,  in  feet. 

The  preceding  deductions  are  based  upon  the  assumption  that  there  is  no 
resistance  to  the  movement  of  the  air.  Such  a  condition  evidently  cannot 
exist  in  the  operation  of  the  ordinary  chimney.  The  motion  of  the  gases 
creates  a  certain  back  pressure  due  to  their  friction  on  the  inner  surface  of  the 
chimney.  This  back  pressure  must  be  deducted  from  that  due  to  the  difference 
in  density  in  order  to  ascertain  the  effective  pressure  which  may  be  applied  to 
compel  the  air  to  pass  through  a  boiler  furnace  and  up  the  chimney. 

The  effective  pressure  which  in  any  given  case  is  necessary  to  overcome  the 
factional  and  other  resistances  of  the  air  in  its  passage  through  the  fuel,  the 
boiler  tubes  and  other  portions  of  the  boiler,  and  impart  to  the  gases  the  neces- 
sary velocity  of  entrance  and  exit,  must  depend  upon  the  existing  conditions  of 
character  of  fuel,  design  and  proportions  of  boiler,  and  other  considerations 
which  render  exact  determination  extremely  difficult. 

Chimney  Design.  —  From  the  preceding  it  is  obvious  that  a  chimney  must  be 
so  designed  as  to  create  sufficient  draft  or  pressure  difference  to  overcome  all 
resistances,  and  in  addition  impart  the  necessary  velocity  to  the  required 
amount  of  air.  The  number  of  pounds  of  coal  which  can  be  burned  in  a  given 


1 82  MECHANICAL   DRAFT. 

time  on  a  given  grate  equals  the  weight  of  air  forced  through,  divided  by  the 
number  of  pounds  of  air  required  for  the  combustion  of  a  pound  of  coal  under 
the  given  conditions.  As  already  shown,  this  latter  amount  may  vary  all  the 
way  from  that  theoretically  necessary  up  to  an  amount  100  per  cent  or  more  in 
excess  thereof.  The  weight  of  the  air  forced  through  in  the  given  period  of 
time  is  equal  to  the  area  through  which  it  is  admitted,  multiplied  by  its  velocity 
and  by  its  density  or  weight  per  unit  of  volume.  To  impart  this  velocity,  and 
also  that  through  the  furnace,  boiler  tubes,  smoke  flue  and  chimney,  there  is  nec- 
essary a  pressure  which  may  be  approximately  ascertained  by  calculations  based 
upon  theoretical  considerations.  The  additional  pressure  required  to  overcome 
the  various  resistances  does  not,  however,  admit  of  the  theoretical  determina- 
tion, and  can  only  be  found  by  direct  experiment,  as  has  already  been  pointed 
out.  These  resistances  are  proportional  to  the  square  of  the  actual  velocity, 
and  depend  on  the  diameter  and  length  of  tubes,  flues  and  chimney,  the  thick- 
ness of  the  fuel  and  its  state  of  division.  It  is,  therefore,  obvious  that  the 
proper  design  of  a  chimney  to  meet  given  conditions  must  be  based  upon  the 
results  of  experiments  under  similar  conditions.  This  fact  readily  accounts  for 
the  great  divergence  in  the  formulae  which  have  been  presented,  and  for  the 
necessity  of  a  series  of  constants  practically  determined  for  application  under 
stated  conditions. 

The  earlier  formulae  deduced  by  Peclet,  Rankine,  Morin  and  Weisbach  have 
until  recently  been  generally  accepted,  but  the  theory  upon  which  they  are 
based  has  of  late  been  the  object  of  considerable  criticism.  Peclet  represented 
the  law  of  draft  by  the  formula  — 

h  =  1  (i  +  G  x  £') 
zg  v  m  } 

In  which  h •'=  "head"  or  height  of  hot  gases  which,  if  added  to  the  column  of 
gases  in  the  chimney,  would  produce  the  same  pressure  at  the 
furnace  as  a  column  of  outside  air,  of  the  same  area  of  base, 
and  height  equal  to  that  of  the  chimney. 

u  =  required  velocity  of  gases  in  chimney. 

G  =  a  constant  to  represent  resistance  to  the  passage  of  air  through 
the  coal. 

/  =  length  of  the  flues  and  chimney. 

m  =  mean  hydraulic  depth,  or  the  area  of  cross-section  divided  by  the 
perimeter. 

/=  a  constant  depending  upon  the  nature  of  the  surfaces  over  which 
the  gases  pass,  whether  smooth,  or  sooty  and  rough. 


MECHANICAL   DRAFT.  183 

Rankine's  and  the  other  formulas  are  somewhat  similar  in  form.  The 
impossibility  of  assigning  proper  values  to  the  constants  in  these  formulae  has, 
up  to  the  present  time,  prevented  their  practical  application  for  chimney 
design,  and  resort  has,  as  a  consequence,  been  made  in  most  cases  to  empirical 
methods. 

Based  upon  this  theory,  however,  the  usual  formula  for  determining  the  num- 
ber of  pounds  of  coal  which  may  be  burned  per  hour,  under  the  conditions  of 
the  draft  which  may  b'e  created  by  a  given  chimney,  takes  the  simple  and  prac- 
tical form  of  — 


In  which  F  '=  number  of  pounds  burned  per  hour. 
A  =  area  of  cross-section  of  chimney. 
H  '  =  height  of  chimney. 
C  =  coefficient  varying  from    ro  to  20,  according  to  the  conditions. 

As  it  stands,  the  formula  implies  that  it  is  a  matter  of  indifference  in  regard  to 
the  draft  whether,  for  instance,  —  other  things  remaining  the  same,  —  a  chimney 
is  built  8  1  feet  high  and  2  feet  square  inside,  or  16  feet  high  and  3  feet  square 
inside  ;  for  in  either  case  the  product  A  */^ff  is  the  same.  Common  sense  at 
once  proves  the  impracticability  of  such  a  formula  thus  broadly  applied.  It  is, 
therefore,  usually  qualified  by  limiting  its  application  to  cases  when  the  total 
grate  area  is  about  eight  times  that  of  the  chimney.  But  evidently  such  limita- 
tion is  arbitrary,  for  this  ratio  of  grate  to  chimney  area  is  not  constant  for  all 
plants. 

The  generally  accepted  empirical  formulae  of  Mr.  William  Kent,1  which  have 
been  very  generally  adopted  in  the  design  of  chimneys,  are  based  upon  the 
observed  conditions  in  a  large  number  of  boiler  plants,  and  take  the  general 
primary  form  of  — 


In  which  A  =  area  of  chimney. 
h  =  height  of  chimney. 
F  =  pounds  of  coal  burned  per  hour. 


'Transactions  American  Society  of  Mechanical  Engineers,  Vol.  VI.,  and  Mechanical  Engi- 
neers' Handbook,  William  Kent,  New  York,  1895. 


1  84  MECHANICAL   DRAFT. 

The  basis  data  are  — 

1.  The  draft  power  of  the  chimney  varies  as  the  square  root  of  the  height. 

2.  The  retarding  of  the  ascending  gases  by  friction  may  be  considered  as 
equivalent  to  a  diminution  of  the  area  of  the  chimney,  or  to  a  lining  of  the 
chimney  by  a  layer  of  gas  which  has  no  velocity.     The  thickness  of  this  lining  is 
assumed  to  be  '2  inches  for  all  chimneys,  or  the  diminution  of  area  equal  to  the 
perimeter  multiplied  by  2  inches  (neglecting  the  overlapping  of  the  corners  of 
the  lining).     For  simplifying  calculation,  the  coefficient  is*  taken  as  the  same  for 
square  and  round  chimneys,  making  the  effective  area  E,  as  expressed  by  the 
equation,  — 

E  =  A  —  0.  ~ 


In  which  A  =  total  area. 

3.  The  power  varies  directly  as  this  effective  area  E. 

4.  A  chimney  should  be  proportioned  so  as  to  be  capable  of  giving  sufficient 
draft  to  cause  the  boiler  to  develop  much  more  than  its  rated  power,  in  case  of 
emergencies,  or  to  cause  the  combustion  of  5  pounds  of  fuel  per  rated  horse- 
power of  boiler  per  hour. 

5.  The  power  of  the  chimney  varying  directly  as  the  effective  area  E,  and  as 
the  square  root  of  the  height  H,  the  formula  for  horse-power  of  boiler  for  a 
given  size  chimney  will  take  the  form,  H.  P.  =  CE  *J~ff  in  which  C  is  a  con- 
stant.    Adopting  the  general  value  for  C  of  3.33,  as  determined  from  the  results 
of  numerous  examples  in  practice,  the  formula  for  horse-power  becomes  — 


from  which  the  values  of  E  and  H  may  also  be  obtained.  In  proportioning 
chimneys  by  these  formulas  the  height  is  generally  first  assumed,  with  due  con- 
sideration of  the  conditions,  and  then  the  area  for  the  assumed  height  and 
horse-power  is  calculated.  The  results  of  calculation  for  all  ordinary  dimen- 
sions of  chimney  and  ranges  of  power  are  presented  in  Table  No.  108. 

The  capacity  in  horse-power  of  a  given  chimney  is  inversely  proportional  to 
the  amount  of  coal  required  per  horse-power.  Therefore,  in  large  plants,  where 
the  economy  of  coal  consumption  is  high,  the  capacities  given  in  the  table  will 
be  proportionably  increased. 

Prof.  H.  B.  Gale,1  having  satisfied  himself,  both  by  experiment  and  mathemat- 
ical analysis  of  the  incorrectness  of  the  common  theory  of  chimney  draft  upon 


iTheory  and  Design  of  Chimneys.      Horace  B.  Gale.     Transactions  American  Society  of 
Mechanical  Engineers,  Vol.  XI. 


MECHANICAL    DRAFT. 


'85 


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1 86  MECHANICAL   DRAFT. 

certain  points,  substitutes  a  theory  in  which  the  height  is  dependent  upon  the 
stack  temperature  and  the  rate  of  combustion,  and  whose  practical  application 
is  based  upon  certain  experimentally  determined  constants. 

He  divides  the  problem  of  chimney  design  into  two  parts :  "  first,  that  of 
ascertaining  the  draft  pressure  necessary  to  burn  the  desired  quantity  of  fuel  in 
the  furnace  ;  second,  that  of  determining  the  dimensions  of  a  chimney  which 
will  produce  the  required  draft  at  the  least  expense."  The  mean  velocity,  v, 
with  which  the  air  enters  the  various  openings  to  the  furnace  he  shows  to  be 
equivalent  to  — 

fi  =    Ta  BF 
1 40,400  a 

In  which  Ta  =  absolute  temperature  of  outer  air, 

B  =  number  of  pounds  of  air  supplied  per  pound  of  fuel, 

F  =  number  of  pounds  burned  per  hour, 

a  =  total  area  of  openings  through  which  air  is  admitted  to  the  fire, 

when  the  relative  density  of  the  external  air  and  the  gases  at  the  same  tempera- 
ture is  in  the  proportion  of  40  to  39.  As  the  effective  pressure  varies  as  the 
square  root  of  the  corresponding  velocity,  the  actual  pressure  difference,  or 
frictional  resistance  P,  is  expressed  by  — 

P  =  Kv* 

In  which  K  =  aggregate  coefficient  of  resistance  of  fire  grate,  bed  of    coals, 
combustion  chamber,  boiler  tubes,  flue  and  chimney. 

Prof.  Gale  has,  by  somewhat  limited  experiments,  established  the  value  of  K 
as  approximately  0.2  for  average  conditions,  but  this  value  varies  greatly  in 
different  cases. 

If  in  the  expression  P  =  KV*  the  value  of  v  (already  given)  be  substituted, 
the  equation  becomes  — 

P-K(    TaBF  V 
\  140,400^' 

which  indicates  the  reduction  of  pressure  in  pounds  per  square  foot  required 
at  the  bottom  of  the  chimney  which  is  to  burn  F  pounds  of  fuel  per  hour,  in  a 
furnace  having  an  area  for  air  admission  a  (in  square  feet),  and  a  coefficient  of 
resistance  A",  allowing  B  pounds  of  air  per  pound  of  fuel. 

The  chimney  height,  If,  to  produce  this  draft  pressure  is  shown  to  be — 

KTS  F 


*~ 
533 


MECHANICAL   DRAFT.  187 

In  which  Ts  =  absolute  temperature  of  chimney  gases. 
M  =  inside  perimeter  of  chimney,  in  feet. 
A  =  area  of  cross-section. of  chimney,  in  square  feet; 

and  the  other  designations  remain  as  in  the  preceding  formulae. 

In  this  formula  allowance  is  made  for  difference  in  density  of  external  air 
and  chimney  gases,  and  in  the  form  of  cross-section.  The  temperature  of  the 
external  air  is  assumed  at  59°,  and  the  most  economical  proportions  of  a 
chimney  taken  to  be  such  that  its  cost  is  proportional  to  the  \  power  of  the  area 
multiplied  by  the  %  power  of  the  height. 

From  the  last  formula  is  ultimately  derived  the  expression  — 

A  =  o. 


as  an  approximate  formula  for  determining  the  most  economical  area  for  a 
chimney  which  is  to  burn  ^pounds  of  coal  per  hour.  For  chimneys  of  ordinary 
proportions  the  sectional  area  in  square  feet  should  by  this  formula  be  approx- 
imately equal  to  the  number  of  pounds  of  fuel  to  be  burned  per  minute. 

For  those  cases  in  which  the  temperature  of  the  chimney  gases  is  between 
150°  and  600°  the  formula  for  height  may  be  reduced  to  — 

rr  K    /    F  Y 

H =  100  _  (  —  ) 
/   V  a  ' 

In  which  /  =  absolute  temperature  of  chimney  gases. 

This  formula  may  be  still  further  simplified  by  adopting  a  value  for  a  equal 
to  one-third  of  the  grate  area,  making  K =  0.2,  and  letting  G  represent  the  area 
of  the  grate  in  square  feet,  whereby  — 

»-.  -;(-§)' 

In  this  formula  the  height  of  chimney  is  proportional  to  the  square  of  the  rate 
of  combustion  per  square  foot  of  grate  divided  by  the  common  temperature  of 
the  chimney  gas.  It  is  to  be  considered,  however,  as  only  a  rough  approxima- 
tion. The  value  of  K  above  must  of  necessity  be  largely  affected  by  the 
character  of  the  fuel,  and  further  experiment  is  necessary  to  determine  its  value 
under  given  conditions. 

Prof.  C.  A.  Smith's  formulae  are  — 

0.0825^ 


1 88  MECHANICAL   DRAFT. 

in  which  the  letters  have  the  same  designation  as  in  the  preceding  formulae. 
These  are  of  the  same  form  as  Kent's,  but  with  a  different  value  for  the  con- 
stant. As  a  consequence,  Smith's  formula  for  area  gives,  for  a  stated  height, 
larger  results  than  Kent's  for  small  fuel  consumption ;  while  for  a  large  con- 
sumption the  area  by  Smith's  formula  is  greater  than  that  by  Kent's.  The 
latter  possesses  the  advantage  of  recognizing  the  practical  fact  that  for  larger 
powers  the  area  of  chimney  required  per  horse-power  becomes  less. 

The  difficulties  in  the  way  of  a  purely  theoretical  consideration  of  the  subject 
of  chimney  draft  have  been  pointedly  considered  by  Mr.  William  Kent,1  who 
concludes  with  these  words :  "  In  the  present  state  of  our  knowledge  I  do  not 
think  any  satisfactory  theory  of  chimneys  can  be  framed  which  will  include  a 
consideration  of  all  the  different  variables  that  affect  the  rate  of  combustion, 
and  from  which  a  formula  can  be  derived  that  will  prove  of  value  in  practice. 
.  .  .  The  best  chimney  formula  that  can  be  obtained  at  present  is  an  empirical 
one,  which  may  be  modified  or  divided  into  two  or  more  to  meet  different 
practical  conditions,  as  new  data  are  obtained  from  experiment." 

The  preceding  discussion  is  sufficient  to  show  the  difference  in  formulae  and 
results,  but  is  evidence  that  they  would  be  in  substantial  accord  if  the  constants 
could  be  based  upon  sufficiently  exhaustive  tests  on  the  amount  of  grate,  fuel 
and  boiler  resistances  under  all  conditions  of  practice  and  qualities,  kinds  and 
conditions  of  fuel. 

Evidently,  the  latter  factor,  that  of  the  fuel,  as  has  already  been  shown  in  the 
discussion  of  draft,  is  the  most  important  of  all,  and  any  formula  which  does 
not  make  proper  allowance  for  variation  in  it  is  liable  to  give  uncertain  results. 
Kent's  constant  is  based  upon  an  assumption  of  fair  practice,  which  reduces  to 
15  pounds  of  coal  consumed  per  square  foot  of  grate  per  hour,  under  the  draft 
created  by  a  chimney  80  feet  high  and  42  inches  in  diameter.  But  the  character 
of  the  coal  is  not  specified. 

Prof.  W.  P.  Trowbridge,2  basing  his  calculation  on  somewhat  different  data, 
determined  the  average  rates  of  combustion  for  various  heights  of  chimney, 
as  here  presented  in  Table  No.  109.  The  air  supply  per  pound  of  fuel  was 
assumed  at  250  cubic  feet  for  all  rates,  and  no  attempt  was  made  to  indicate 
the  kind  of  fuel  or  the  effect  of  any  change  in  its  character.  It  will  be  noticed 
that  this  table  gives  a  rate  of  16.9  pounds  for  an  So-foot  chimney,  as  com- 
pared with  Kent's  figure  of  15  pounds;  while  Table  No.  105,  presented  in 
the  preceding  chapter,  makes  the  rate  19  pounds. 


'  Transactions  American  Society  of  Mechanical  Engineers,  Vol.  XL,  p.  995. 
2  Heat  and  Heat  Engines.     W.  P.  Trowbridge.     New  York,  1874. 


MECHANICAL    DRAFT. 


189 


Table  No.   109.  — Height  of  Chimney  to  Produce  Certain  Rate  of  Combustion. 


Height  of  Chimney  in  Feet. 

Pounds  of  Coal  Burned 
per  Hour  per  Square  Foot  of  Cross- 
Section  of  Chimney. 

Pounds  of  Coal  Burned  per  Hour 
per  Square  Foot  of  Grate,  the  Ratio  of 
Grate  to  Cross-Section  of  Chimney 
beingS  to  i. 

20 

60 

7-5 

25 

68 

8-5 

3° 

76 

9-5 

35 

84 

10.5 

40 

93 

n.6 

45 

99 

12.4 

So 

»°S 

I3-1 

55 

in 

13.8 

60 

116 

M-S 

65 

121 

*$•* 

70 

126 

15-8 

75 

J31 

16.4 

80 

J35 

16.9 

85 

139 

17.4 

90 

144 

1  8.0 

95 

148 

18.5 

100 

152 

19.0 

'°5 

156 

19-5 

no 

1  60 

20.0 

Prof.  R.  H.  Thurston1  gives  this  rough  rule  for  the  case  of  anthracite  coal : 
"  Subtract  i  from  twice  the  square  root  of  the  height,  and  the  result  is  the  rate 
of  combustion."  This  also  gives  the  rate  for  an  8o-foot  chimney  as  16.9. 

As  changes  in  the  character  of  the  fuel  are  likely  to  be  made  at  any  time  in 
the  life  of  a  boiler  plant,  it  is  obvious  that  the  chimney  height,  which  is  a 
measure  of  the  draft  pressure,  should  be  made  sufficient  to  meet  all  require- 
ments. How  much  effect  the  kind  of  fuel  may  have  upon  the  height  of  chimney 
necessary  for  its  combustion  is  evidenced  in  the  results  of  extensive  tests  with 
telescopic  stacks  by  J.  J.  Kinder.  He  found  that,  for  substantially  equivalent 
results,  the  height  for  free-burning  bituminous  coals  should  be  75  feet,  for  slow- 
burning  bituminous  115  feet,  and  for  fine  anthracite  coals  125  to  150  feet. 

These  results  point  most  clearly  to  the  necessity,  as  alreadly  stated,  of  first 
determining  the  height  of  the  chimney  necessary  to  burn  a  given  kind  of  fuel 


Manual  of  Steam  Boilers.     R.  H.  Thurston.     1888. 


1 9o  MECHANICAL    DRAFT. 

at  a  stated  rate  per  square  foot  of  grate,  and  then  making  it  of  sufficient  area 
to  burn  the  requisite  amount.  Evidently,  in  the  light  of  common  experience,  as 
plainly  shown  by  Mr.  Kinder's  tests,  the  first  cost  and  the  fixed  charge  for  the 
necessary  chimney  will  vary  with  the  kind  of  coal  used.  The  existing  relation 
may  be  approximately  illustrated  by  a  consideration  of  Table  No.  108.  It 
is  there  indicated  that  245  horse-power  of  boilers  may  be  served  either  by  a 
chimney.  90  feet  high  and  42  inches  in  diameter,  or  by  one  125  feet  high  and  39 
inches  in  diameter.  Such  differences  are  well  within  the  limits  for  different 
kinds  of  coal,  as  given  above.  If,  then,  in  one  case  the  coal  can  be  burned 
with  the  draft  produced  by  a  go-foot  chimney,  while  in  another  a  different  kind 
of  coal  requires  a  chimney  125  feet  high,  the  difference  in  the  cost  of  the  two 
chimneys  must  enter  as  an  important  item  in  the  question  of  ultimate  economy. 
Upon  the  basis  established  by  Prof.  Gale,  and  previously  referred  to,  the 
cost  of  a  chimney  is  nearly  proportional  to  the  J  power  of  its  area  multiplied  by 
the  £  power  of  the  height.  As  the  area  is  proportional  to  the  square  of  the 
diameter,  the  relative  costs  in  the  case  of  these  chimneys  become  — 

Cost  of  go-foot  chimney  =  42 3  x  90?=  10,316. 
Cost  of  125-foot  chimney  =  39?  x  1252=16,072. 

That  is,  the  chimney  would  cost  about  56  per  cent  more  in  one  case  than  in 
the  other  to  secure  the  same  result  in  boiler  power. 

As  regards  the  capacity  of  a  chimney  of  a  given  height,  which  is  directly 
dependent  upon  its  area,  it  is  necessary  to  make  it  originally  sufficient  for  all 
future  probabilities.  If  this  capacity  be  made  too  great  for  present  conditions, 
it  becomes  necessary  to  reduce  the  volume  by  means  of  dampers.  But  by  no 
means,  in  the  case  of  a  chimney,  can  the  draft  itself,  as  measured  by  pressure 
difference  and  power  to  overcome  resistances,  be  increased  above  that  normally 
due  to  its  height  with  given  temperature  conditions. 

Thus,  for  instance,  if  with  a  given  chimney  the  grate  area  be  reduced  and 
the  rate  of  combustion  proportionally  increased  so  as  to  maintain  the  same  total 
consumption,  the  resistances  will  be  increased  because  of  thicker  fires  and 
decreased  free  area  through  them.  To  overcome  these  resistances  there  is 
demanded  greater  draft,  but  as  the  draft  of  a  chimney  is  absolutely  limited  by 
its  height,  any  further  expenditure  of  its  draft  for  this  purpose  must  by  just  so 
much  reduce  the  portion  of  its  draft  available  for  producing  the  requisite  air 
flow.  If  the  chimney  be  large  for  the  boiler  plant,  and  if  under  the  lower  com- 
bustion rate  it  be  necessary  to  operate  it  with  practically  closed  dampers,  there 
may  be  sufficient  reserve  to  meet  the  requirements.  But  under  ordinary  condi- 
tions the  chimney  has  but  little  power  to  respond  to  such  requirements,  or  even 


MECHANICAL   DRAFT. 


191 


to  a  material  increase  of  the  rate  of  combustion  upon  the  regular  grates.  That 
is,  its  ability  and  capacity  are  distinctly  limited,  and  to  provide  against  con- 
tingencies it  is  usually  necessary  to  build  and  pay  the  fixed  charges  on  a 
structure  more  expensive  than  is  regularly  required. 

On  the  other  hand,  with  mechanical  draft  produced  by  means  of  a  fan,  as 
ordinarily  applied,  any  increase  in  the  resistances  automatically  increases  the 
pressure  difference  which  is  created  by  the  fan  up  to  the  limit  of  its  capacity, 
as  will  be  explained  at  length  in  a  subsequent  chapter.  In  other  word,  whereas 
with  a  chimney  increased  resistances  decrease  the  pressure  difference,  the 
exact  reverse  is  the  tendency  with  a  fan.  As  it  has  already  been  shown  that  in 
ordinary  boiler  practice  almost  all  of  such  difference  in  pressure  is  required  to 
overcome  these  resistances,  the  special  adaptability  of  the  fan  for  draft  produc- 
tion cannot  fail  to  be  obvious. 

Efficiency  of  Chimneys.  —  The  chimney  as  a  means  of  creating  a  movement 
of  air  depends  upon  the  heating  of  that  air ;  although,  as  shown,  its  actual 
movement  is  to  be  considered  upon  purely  mechanical  grounds.  The  heat  thus 
employed  is,  however,  absolutely  wasted,  so  far  as  its  utilization  for  any  other 
purpose  is  concerned.  Any  attempt  to  extract  more  of  the  heat  from  the  gases 
as  they  escape  from  the  boiler  must  result  in  a  reduction  of  the  draft.  This 
inherent  loss  is,  therefore,  always  chargeable  to  any  plant  in  which  the  draft  is 
produced  by  a  chimney,  and  possibilities  in  the  way  of  increased  economy  must 
relate  only  to  other  losses  so  long  as  a  given  chimney  is  retained. 

The  percentage  of  the  total  calorific  value  of  coal  which  is  carried  off  by  the 
products  of  combustion,  and  therefore  available  only  for  the  production  of  draft, 
has  already  been  presented  in  Table  No.  52  for  different  degrees  of  excess  of 
air  and  of  temperature  above  the  atmosphere.  As  there  shown,  this  loss 
actually  amounts  to  19  per  cent  when  the  gases  are  at  500°  and  the  excess  of 
air  is  100  per  cent.  Evidently  such  a  great  loss  as  is  thus  possible  should 
require  energetic  effort  to  secure  its  reduction  by  a  more  economical  substitute 
for  the  chimney. 

Of  course  the  weight  of  air  moved  by  means  of  a  chimney  of  given  height 
must  depend  upon  its  area.  As  heat  is  the  means  by  which  this  air  movement 
is  brought  about,  the  efficiency  of  the  chimney  must  be  measured  by  the  amount 
of  heat  expended  for  this  purpose.  Heat  being  transformable  into  work,  the 
efficiency  is,  therefore,  to  be  measured  by  the  number  of  foot-pounds  of  work 
represented  by  the  pressure  difference  exerted  through  the  distance  represented 
by  the  height  of  the  column  of  cold  air  necessary  to  produce  the  given  pressure, 
as  compared  with  the  number  of  foot-pounds  represented  by  the  total  amount 
of  heat  expended. 


192  MECHANICAL   DRAFT. 

Suppose,  for  the  purposes  of  illustration,  a  chimney  100  feet  high,  having  a 
cross-sectional  area  of  10  square  feet,  the  atmospheric  temperature  at  62°  and 
the  temperature  of  the  chimney  gases  at  500°;  and  further,  for  simplicity, 
assume  that  no  work  is  lost  in  friction  and  that  heated  air  is  substituted  for 
the  hot  gases,  for  their  density  and  specific  heat  are  approximately  the  same. 
Under  these  conditions  the  density  d  at  62°  will  be  0.0761  pounds  per  cubic 
foot,  and  the  density  //',  0.0414  at  500°.  Therefore,  by  the  formula  previously 
given  the  pressure  difference  with  the  chimney  100  feet  high  will  be  — 

P  =  h(d—  d^. 

=  100  (0.0761  —  0.0414). 

=  3.47  pounds  per  square  foot. 

This  makes  the  total  pressure  difference  3.47  x  10  =34.7  pounds  over  the 
entire  area  of  the  chimney. 

The  height  of  a  column  of  external  air  which  will  produce  the  above  pressure 
per  square  foot  is  — 


/o. 

-    ioo(  -  —  - 

V  0.0761 

=  45.6  feet. 
The  velocity  of  the  air  entering  the  base  of  the  chimney  under  this  head  is  — 


v  =  V2gff=  V  64.32  X  45-6 

=  54.2  feet  per  second; 
and  its  weight  per  second,  — 

Weight  =  54.2  x  10  x  0.0761  =  41.25  pounds. 

The  movement  of  this  air  is  the  result  of  heating  it  from  62°  to  500°;  that  is, 
through  500  —  62  =438°.  As  the  specific  heat  of  air  under  constant  pres- 
sure is  0.2375,  the  total  heat  expended  per  second  in  moving  41.25  pounds  is  — 

Heat  expended  =  41.25  x  438  x  0.2375 
=  4,291.0  B.T.U. 

As  one  heat  unit  is  equivalent  to  778  foot-pounds  of  work,  the  work  equivalent 
to  the  total  amount  of  heat  expended,  and  which  goes  to  waste  without  per- 
forming useful  work  in  heating  the  water,  is  — 

Work  equivalent  of  heat  =  4,291.0  x  778 

=  3>338>398  foot-pounds. 


MECHANICAL    DRAFT. 


193 


But  the  work  actually  done  is  the  result  of  overcoming  a  total  pressure  of  34.7 
pounds  through  a  distance  of  54.2  feet;  that  is,  — 

Work  actually  done  =  34.7  x  54.2  =  1880.7  foot-pounds. 
Therefore,  the  efficiency  of  the  chimney  is  — 


That  is,  less  than  six  ten-thousandths  of  the  heat  expended  is  represented  by 
the  work  done.  In  practice  the  resistance  of  the  chimney,  the  cooling  of  the 
gases  in  their  passage  up  it  and  other  causes  combine  to  decrease  even  this 
extremely  low  efficiency. 

If  in  the  place  of  the  chimney  there  be  substituted  a  fan  of  proper  size, 
arranged  to  be  driven  by  a  direct-connected  engine,  the  efficiency  with  which  it 
would  move  the  above-stated  volume  of  air  under  the  given  conditions,  but 
without  the  chimney,  may  be  calculated  with  reasonable  accuracy.  Evidently 
the  efficiency  of  a  fan  thus  applied  is  the  resultant  of  the  efficiencies  of  the 
steam  boiler,  the  engine  and  the  fan,  together  with  the  loss  by  friction  in  the 
apparatus.  If  the  combined  efficiency  of  the  boiler  and  engine  be  taken  as  one- 
tenth,  the  efficiency  of  the  fan  at  the  low  value  of  only  five-tenths  and  the  loss 
from  friction  as  two-tenths,  or  the  efficiency  as  regards  friction  eight-tenths,  the 
resulting  efficiency  of  the  system  will  be  — 

Efficiency  =  o.i  x  0.5  x  0.8  =  0.04. 

That  is,  of  the  work  done,  or  its  equivalent  in  heat  units  expended  to  produce 
a  given  result,  one  twenty-fifth  is  actually  applied  for  that  purpose;  the  re- 
mainder is  lost  in  the  processes  of  transformation  and  transmission  and  in 
friction.  This  efficiency,  which  allows  for  loss  by  friction,  as  was  not  the  case 
with  the  chimney,  is  — 

0.04 


0.000563 

times  greater  than  that  of  the  chimney. 

It  may  be  shown  that  the  relative  efficiency  of  a  fan  and  a  chimney  is  depen- 
dent upon  the  height  of  the  chimney  and  not  upon  the  difference  in  temperature, 
and  that  it  varies  inversely  as  the  height  of  the  chimney.  Thus  in  the  case 
of  a  chimney  75  feet  high  the  fan  would,  upon  the  same  basis  as  above,  have 
an  actual  efficiency  of  — 


194 


MECHANICAL   DRAFT. 


times  greater  than  the  theoretical  efficiency  of  the  chimney,  while  at  200  feet 
high  the  fan  would  still  have  35.5  times  higher  efficiency;  but  this  improvement 
in  favor  of  the  chimney  would  be  largely  offset  by  its  proportionally  greater  cost 
as  compared  with  a  fan. 

All  other  questions  aside,  the  fan  is,  therefore,  above  question  far  more  eco- 
nomical than  the  chimney.  This  economy  means  that  the  surplus  heat  can  be 
utilized  and  the  gases  reduced  to  a  minimum  temperature  before  they  enter  the 
fan. 

Says  Mr.  W.  H.  Bryan,1  in  discussing  the  advantages  of  mechanical  draft: 
"  As  a  general  rule,  however,  it  may  be  safely  stated  that  in  this  year  of  our 
Lord,  1896,  the  building  of  tall  chimneys  to  secure  draft  simply  advertises  the 
owner's  lack  of  familiarity  with  modern  improvements  or  his  want  of  confidence 
in  results  easily  demonstrated." 

The  following  expression  of  opinion  by  the  well-known  engineer,  Mr.  William 
O.  Webber, 2  is  most  emphatic :  "  I  wish  to  go  on  record  as  advocating  mechan- 
ical draft,  as  having  the  following  advantages  :  Less  first  cost,  and  the  larger 
the  plant  the  greater  the  proportion  of  this  saving  of  first  cost ;  secondly,  the 
elasticity  of  this  system,  as  any  given  size  of  fan  and  stack  can  be  used  for  two 
or  three  times  its  original  capacity,  simply  by  the  speeding-up  of  the  draft  fan ; 
and,  third,  the  fact  that  this  induced  draft  is  absolutely  controllable  and  is 
regulated  by  the  pressure  of  steam  in  the  boilers  and  the  demand  upon  the 
boilers  for  steam ;  and,  finally,  it  is  not  .influenced  by  atmospheric  conditions. 
I  think  that  in  this  latter  connection  few  people  realize,  unless  they  have  spent 
considerable  time  in  a  boiler  room,  how  the  barometric  and  wind  pressures  affect 
the  draft  of  ordinary  brick  chimneys,  and  that  it  is  more  strongly  noticeable 
where  an  independent  small  metallic  stack  is  used  for  each  boiler  of  sufficient 
height  to  make  a  draft  in  the  ordinary  manner. 

"The  advantages  in  favor  of  the  mechanical  draft  are,  that  no  matter  what 
the  weather  conditions  are,  as  good  a  draft  can  be  obtained  at  one  time  as 
another. 

"This  system  has  been  in  use  long  enough  now  to  have  demonstrated  its 
practicability  and  economy,  and  in  my  opinion  the  old,  tall,  expensive  brick 
chimney  is  out  of  date." 


1  Boiler  Efficiency,  Capacity  and  Smokelessness,  with  Low-Grade    F"uels.     Wm.  H.  Bryan. 
A  paper  read  before  the  Engineers'  Club  of  St.  Louis,  Oct.  21,  1896. 

2  Large  Brick  Chimneys  versus  Mechanical  Draft  and  Small  Stacks.     William  O.  Webber. 
Power,  New  York,  July,  1897. 


CHAPTER  X. 
MECHANICAL  DRAFT. 

Definition.  —  The  chimney  has  long  stood  as  practically  the  only  available 
means  of  producing  draft,  which,  thus  produced,  has  been  commonly  called 
"  natural  draft."  If  the  chimney  satisfactorily  met  all  of  the  requirements  for 
modern  boiler  practice,  one  would  scarce  expect  to  see  a  substitute  proposed. 
But  the  very,  substitution  of  another  means  is  the  strongest  evidence  of  the 
acknowledged  inadequacy  of  the  chimney,  not  to  produce  draft,  but  to  do  it  as 
satisfactorily  and  economically  as  the  substitute. 

Primarily  introduced  for  the  purpose  of  increasing  the  rate  of  combustion,  arti- 
ficial draft  was  designated  as  ''forced  draft,"  its  field  of  application  being  con- 
sidered to  begin  where  that  of  the  chimney  ended.  By  later  refinements  it  has, 
however,  become  not  only  a  means  of  assisting  chimney  draft,  and  of  produc- 
ing the  conditions  requisite  to  accelerated  combustion,  but  it  is  now  accepted  as 
a  convenient  and  efficient  substitute  for  the  chimney  under  all  ordinary  condi- 
tions. The  extent  of  its  success  and  adoption  must  evidently  be  a  measure  of 
such  convenience  and  efficiency. 

Artificial  draft  may  be,  and  has  been,  produced  by  means  of  steam  jets  induc- 
ing a  flow  of  air,  by  blowing  engines,  by  air  compressors,  by  positive  rotary 
blowers  and  by  fan  blowers  or  exhausters.  Although  the  practical  success  of 
the  locomotive  is  largely  due  to  Stephenson's  introduction  of  the  steam  nozzle 
for  draft  production,  it  does  not  follow  that  the  same  method  is  applicable 
where  the  exhaust  steam  would  not  otherwise  be  wasted.  The  blowing  engine, 
the  air  compressor  and  the  rotary  positive  blast  blower  all  possess  disadvan- 
tages which  render  undersirable  their  adoption  for  this  purpose.  In  fact,  they 
have  been  introduced  to  only  a  very  limited  extent.  The  centrifugal  fan  has, 
however,  been  most  extensively  applied  under  all  conceivable  conditions,  until 
it  has  become  the  symbol  of  artificial,  or,  as  it  may  properly  be  designated,  of 
mechanical  draft,  and  is  today  the  accepted  substitute  for  the  chimney. 

Steam  Jets.  —  The  steam  jet  as  a  means  of  inducing  a  flow  of  air  is  usually 
constructed  upon  the  injection  principle.  It  has  been  applied  in  the  chimney 
for  inducing  the  air  movement  through  the  fuel,  as  well  as  in  the  ashpit  for 
forcing  therein  a  volume  of  air  which  is  caused  to  pass  upward  through  the 


196 


MECHANICAL    DRAFT. 


fuel.  In  connection  with  the  latter  arrangement,  steam  jets  have  often  been 
introduced  to  also  deliver  air  above  the  fuel,  frequently  in  the  form  of  a  number 
of  finely  divided  streams  designed  to  mix  intimately  with  the  gases  arising  from 
the  fuel  bed. 

The  introduction  of  steam  in  conjunction  with  the  air,  which  results  from 
the  use  of  the  steam  jet,  is  often  asserted  to  assist  in  keeping  a  fire  free  and 
open,  particularly  in  the  case  of  fine  anthracite  fuels.  But,  in  so  far  as  steam 
for  this  purpose  may  be  necessary,  it  can  be  as  well  introduced  in  connection 
with  a  fan.  Consequently,  the  merits  of  the  steam  jet,  as  compared  with  a  fan, 
must  rest  solely  upon  the  relative  efficiency  with  which  a  given  amount  of  air  is 
supplied ;  or,  as  more  simply  measured,  by  the  proportion  which  the  steam 
required  to  operate  the  steam  jet  or  fan  bears  to  the  total  steam  produced  by 
the  boiler  in  connection  with  which  it  operates.  In  either  case  the  percentage 
of  steam  thus  used  is  largely  dependent  upon  the  size  of  the  plant,  being 
greatest  with  the  smallest  plant. 

Careful  experiments,  conducted  at  the  New  York  Navy  Yard,1  to  determine 
the  best  form  of  steam  jet  for  producing  forced  draft  in  launch  boilers,  served 
to  show  the  inefficiency  of  such  devices  for  this  purpose.  The  results  of  five 
series  of  tests  are  presented  in  Table  No.  no.  A  different  form  of  jet,  indi- 

Table  No.  no.  —  Results  of  Experiments  upon  Steam  Jets  at  New  York  Navy  Yard. 

Pounds  of  Water  Evaporated  per  Hour. 


A 

B 

C 

D 

E 

In  boiler  making  steam        ..... 
In  boiler  supplying  steam  jet       . 

463.8 
97-5 

580.0 

120.0 

361.25 
30.00 

528.5 
63.2 

545-00 

76.25 

Per  cent  of  steam  made  as  used  in  steam  jet 

21.  2O 

20.70 

8.30 

I2.OO 

14.00 

cated  by  a  designating  letter,  was  used  in  each  case ;  its  supply  of  steam  being 
taken  from  a  boiler  separate  from  that  to  which  the  jet  was  applied.  The  per- 
centages of  steam  are  such  —  the  maximum  being  21.2  per  cent  —  as  to  make 
the  adoption  of  a  steam  jet  out  of  the  question  when  any  other  means  of  draft 
production  can  be  employed.  In  jet  C,  which  had  a  hole  only  one-sixteenth 
inch  in  diameter,  and  which  was  the  most  economical  of  all,  the  steam  used 
was  one  pound  in  two  minutes.  The  amount  of  steam  required  by  a  fan 
blower,  as  will  be  shown  later,  is  under  ordinary  conditions  from  a  fraction  of 


Annual  Report  of  the  Chief  of  the  Bureau  of  Steam  Engineering,  U.  S.  Navy.     1890. 


MECHANICAL    DRAFT. 


197 


i  per  cent  up  to  a  possible  maximum  of  3  or  4  per  cent  in  small  boiler  plants  or 
with  uneconomical  apparatus ;  and  practically  the  whole  of  this  expenditure  of 
power,  in  the  form  of  exhaust  steam,  may  be  subsequently  utilized  for  heating 
or  similar  purposes. 

The  case  of  the  steam  jet  may  be  briefly  summarized  thus :  It  has  the  advan- 
tage of  costing  very  little  to  put  in  and  keep  in  repair.  Its  disadvantages  are  : 
first,  it  requires  a  very  large  amount  of  steam  to  run  it;  second,  it  introduces  a 
large  amount  of  water  or  steam,  all  of  which  has  to  be  heated  and  carried  up 
chimney ;  third,  unless  very  carefully  managed  there  is  a  large  development  of 
carbonic  oxide,  hydrogen  and  marsh  gas,  due  to  dissociation  of  the  water, 
which  has  a  tendency  to  carry  off  a  great  deal  of  heat  in  the  stack ;  fourth,  the 
intensity  of  draft  producible  by  this  means  is  distinctly  limited ;  and,  fifth,  and 
by  no  means  least,  the  noise  incident  to  its  use  is  at  times  almost  intolerable. 

The  comparative  effect  of  the  steam  jet  and  the  fan  blower  upon  the  composi- 
tion of  the  chimney  gases  is  well  shown  by  a  test  by  Mr.  Eckley  B.  Coxe,1  upon 
two  adjoining  sets  of  boilers  using  the  same  fuel  and  fired  by  the  same  men,  as 
presented  in  Table  No.  in.  A  was  fitted  with  a  steam  jet  and  B  with  a  fan 
blower  and  Coxe  stoker.  The  losses  indicated  by  the  excessive  presence  of 
carbonic  oxide  (CO),  hydrogen  (H)  and  marsh  gas  (CH4)  in  the  case  of  the 
steam  jet  are  such  as  to  most  emphatically  indicate  its  inefficiency. 

Table  No.  in.  —  Comparative  Gas  Analyses  with  Steam  Jet  and  Fan  Blower. 


§ 

CONSTITUENTS. 

2 

| 

0 

CO 

C02 

H 

CH4 

A 

With  steam  jet   

0.30 

I3-I5 

8.20 

1  1.  08 

2.OO 

B 

With  fan  blower          .... 

1.70 

0.40 

16.80 

The  relative  merits  of  the  fan  and  the  jet  are  thus  expressed  by  Mr.  Coxe : 
"  The  fan  is  more  expensive  to  install  and  may  cost  more  to  keep  in  order,  but 
where  the  arrangements  can  be  made  to  utilize  the  heat  in  the  stack  gases  it  is 
more  economical  so  far  as  heat  units  used  are  concerned.  It  has  one  great 
advantage,—  it  is  possible  to  at  all  times  obtain  the  exact  blast  necessary  to 
produce  the  best  results  in  the  furnace,  which  is  very  important." 


i  Some  Thoughts  upon  the  Economical  Production  of  Steam,  etc.     Eckley  B.  Coxe.     Trans- 
actions New  England  Cotton  Manufacturers'  Association.     1895. 


198  MECHANICAL   DRAFT. 

Fans.  —  The  centrifugal  fan,  or  fan  blower,  as  an  apparatus  for  producing 
draft,  is  no  new  thing.  As  applied  for  the  purposes  of  ventilation  it  dates  back 
to  the  sixteenth  century,  but  as  a  substitute  for  or  auxiliary  to  the  chimney 
its  first  application  appears  to  have  been  made  early  in  the  present  century.  In 
1827,  Edwin  A.  Stevens,  of  Bordentown,  N.  J.,  arranged  a  fan  for  forcing  the 
air  into  the  ashpits  of  the  boilers  on  the  steamer  North  America.  In  conjunc- 
tion with  his  brother,  R.  L.  Stevens,  he  subsequently  experimented  at  consid- 
erable length  with  various  methods  of  applying  the  fan.  It  is  said  that  in  1824 
John  Ericsson  fitted  the  British  steamer  Victory  for  forced  draft  by  means  of  a 
fan,  and  it  is  certain  that  in  1830  the  Corsair  was  so  equipped  by  him. 

But  engine  speeds  and  steam  pressures  were  then  low;  the  demand  for 
accelerated  combustion  was  not  urgent,  and  experience  had  not  been  gained  in 
the  proper  application  of  fans  for  forced  draft.  As  a  consequence  this  eco- 
nomic improvement,  which  was  to  mean  so  much  in  later  days,  was  but  very 
meagrely  adopted. 

The  fact  that  these  first  applications  were  made  on  steam  vessels  indicates  the 
natural  adaptability  of  the  fan  for  the  purpose.  It  is,  therefore,  not  surprising 
to  find  that  the  arrangement  was  again  taken  up,  this  time  by  the  United  States 
during  and  subsequent  to  the  war  of  the  Rebellion.  Extensive  tests  were  con- 
ducted by  Chief  Engineer  B.  F.  Isherwood,  but  there  remained  still  another 
stage  in  its  progress  toward  general  application,  both  on  sea  and  land. 

At  about  this  time  B.  F.  Sturtevant  began  to  manufacture  and  introduce  fans 
of  various  sizes  for  the  acceleration  of  draft  in  stationary  boilers,  and  many  were 
installed  throughout  the  country.  These  were  almost  universally  applied  for 
forcing  the  air  into  the  ashpit,  and  at  once  found  a  ready  market  because  of  the 
advantages  incident  to  their  use  in  the  burning  of  cheap  grades  of  fuel,  which 
had  previously  been  impossible  with  the  ordinary  chimney  draft. 

The  advent  of  the  torpedo  boat  marked  the  further  introduction  of  forced 
draft  for  marine  boilers.  In  these  small,  compact  vessels  tall  stacks  were  out  of 
the  question,  but  strong  draft  and  the  utmost  steaming  capacity  per  ton  of  weight 
were  an  absolute  necessity.  From  success  with  these  smaller  boats  it  was  but  a 
natural  step  to  those  of  larger  size.  In  1877  the  French  government  equipped 
the  advice  boat  La  Bourdonnais  for  forced  draft,  and  in  1882  a  definite  move 
was  made  in  the  British  Navy  by  providing  fans  for  the  production  of  draft  on 
the  Satellite  and  Conqueror. 

Shortly  after,  the  United  States  Navy  again  took  up  this  important  element  in 
the  design  of  the  modern  naval  vessel  and  introduced  forced  draft  upon  all  of 
the  vessels  of  the  "  new  navy."  Of  the  vessels  thus  equipped  almost  all  are 
supplied  with  Sturtevant  fans. 


MECHANICAL    DRAFT. 


199 


From  the  navy  it  was  but  a  natural  step  to  the  merchant  marine.  The 
advantages  of  the  fan  for  draft  production  were  recognized,  and  it  has  been 
extensively  introduced  both  in  the  lake  and  ocean  steamships.  The  form  of 
application  has  varied  greatly.  The  especial  desirability  of  fuel  economy  on 
shipboard  results  in  many  cases  in  the  use  of  special  forms  of  heat  abstractors 
or  air  preheaters,  whose  very  use  is  possible  only  where  a  fan  is  employed  to 
produce  the  draft. 

On  land  the  process  of  development  has,  during  the  past  few  years,  been 
simply  phenomenal.  From  the  under-grate  forced-draft  systems  there  has  been 
a  gradual  change  to  over-grate  induced  systems,  until  the  matter  of  mechanical 
draft  engages  the  attention  of  every  progressive  engineer  in  the  design  of  a 
steam  plant.  The  various  forms  of  application,  together  with  the  types  of  fans 
therefor,  will  be  described  at  length  in  succeeding  chapters. 

Two  types  of  fans  exist.  The  first,  known  as  the  disc  or  propeller  wheel,  is 
constructed  on  the  order  of  the  screw  propellor,  and  moves  the  air  in  lines 
parallel  to  its  axis,  the  blades  acting  upon  the  principle  of  the  inclined  plane. 
The  second,  or  fan  blower  proper,  consists  in  its  simplest  form  of  a  number  of 
blades  extending  radially  from  the  axis  and  presenting  practically  flat  surfaces 
to  the  air  as  they  revolve.  By  the  action  of  the  wheel  the  air  is  drawn  in  axially 
at  the  centre  and  delivered  from  the  tips  of  the  blades  in  a  tangential  direction. 
This  type  may  be  simply  designated  as  the  centrifugal  fan,  or,  more  properly,  as 
the  peripheral  discharge  fan. 

The  propeller,  or  disc  fan,  which  is  available  for  ventilating  purposes  when 
it  acts  against  slight  resistances,  is  practically  useless  as  a  means  of  draft  pro- 
duction. The  desired  results  can  only  be  secured  by  the  use  of  the  peripheral 
discharge  type,  which  for  this  purpose  is  usually  enclosed  in  a  case  of  such 
shape  as  to  provide  free  movement  for  the  air  as  it  escapes  at  the  periphery,  and 
an  outlet  through  which  it  is  all  delivered.  The  detailed  construction  of  these 
fans,  with  illustrations  of  their  various  types,  will  be  indicated  in  the  succeeding 
chapter. 

Although  theoretically  there  should  be  a  difference  in  the  form  of  the  wheels 
designed  for  creating  pressure  and  creating  vacuum,  yet,  in  the  common  accep- 
tation of  the  terms,  the  distinction  between  a  blower  and  an  exhauster  is  pri- 
marily one  of  adaptation  rather  than  of  construction ;  the  normal  use  of  a 
blower  being  to  force  air  into  a  given  space,  while  an  exhauster  is  employed  to 
remove  air  from  an  enclosure.  For  convenience  of  attachment  of  pipe  connec- 
tions, an  exhauster  is  provided  with  an  inlet  on  one  side  only;  while  a  blower, 
being  exempt  from  this  requirement,  is  provided  with  two  inlets,  one  upon 
either  side. 


200  MECHANICAL    DRAFT. 

Theory  of  Fans.  —  In  operation,  the  peripheral  discharge  fan  sets  in  motion 
the  air  within  it,  which,  acting  by  centrifugal  force,  is  delivered  tangentially  at 
the  outer  circumference  of  the  wheel.  Air  rushes  in  at  the  axial  iniet  to  fill  the 
space  between  the  blades,  in  which  there  is,  by  the  centrifugal  action,  a  tendency 
to  form  a  vacuum.  The  degree  of  this  vacuum  is  dependent  upon  the  circum- 
ferential speed  of  the  wheel ;  and  the  velocity  of  the  air  discharged,  through  an 
outlet  of  proper  size,  is  substantially  equal  thereto. 

It  has  already  been  shown  that  a  certain  ideal  head  is  necessary  to  produce 
a  given  velocity.  This  head,  which  may  be  expressed  in  terms  of  the  pressure 

and  density,  as  h  =  ?-,  is  usually  designated  as  the  "head  due  to  the  velocity." 

The  pressure,  of  which  this  head  is  one  of  the  factors,  is  understood  to  be 
that  existing  in  an  enclosed  space  from  which  the  air  escapes  at  a  velocity  ex- 
pressed by  the  formula,  v  =  \/  2<rk  __  I  iff-  •  But  the  pressure  which  this 

V      d 

stream  of  moving  air  may  exert  upon  any  external  surface  with  which  it  comes  in 
contact  may  be  different  from  that  which  existed  in  the  reservoir  and  produced 
the  given  velocity.  This  external  pressure  is  due  to  the  impact  and  reaction  of 
the  air,  and  for  a  given  velocity  depends  in  quantity  upon  the  size  and  form  of 
the  surface  and  the  angle  of  incidence.  Theoretically,  if  the  density  is  con- 
sidered constant,  this  pressure,  in  the  case  of  a  stream  striking  a  plane  surface 
at  right  angles,  will  be  double  that  which  produced  the  velocity ;  or,  more  clearly 
stated,  in  its  relation  to  water,  the  reaction  of  such  a  stream  is  equal  to  the 
weight  of  a  column  of  water  whose  cross-section  is  that  of  the  stream  and  whose 
height  is  double  that  (zfi)  due  to  the  velocity.  If  the  plane  surface  be  of 
proper  dimensions  and  surrounded  by  a  raised  border,  to  prevent  the  ready 
escape  of  the  water,  the  theoretical  pressure  will  be  four  times  the  head  due  to 
the  velocity.  Other  shapes  will  give  other  values.  With  air,  its  lack  of  viscosity 
and  the  partial  vacuum  formed  on  the  back  side  of  the  plate  influence  the 
actual  results. 

In  the  attempt  to  force  air  at  a  given  velocity  through  a  given  pipe,  it  is  the 
province  of  the  fan  wheel,  if  employed  therefor,  to  create  within  the  fan  case 
a  total  pressure  above  the  atmosphere  which  shall  be  sufficient  to  produce  the 
velocity  and  also  overcome  the  resistances  of  the  case  and  the  pipe.  If,  how- 
ever, the  pipe  be  removed  and  the  fan  be  allowed  to  discharge  the  air  through  a 
short  and  properly  shaped  outlet,  the  pressure  necessary  will,  with  an  efficient 
fan,  be  substantially  that  required  to  produce  the  velocity.  The  method  of  de- 
termining the  velocity  due  to  any  given  pressure  has  already  been  explained  and 
the  results  of  calculation  embodied  in  Tables  Nos.  91  and  92.  From  the  same 


MECHANICAL   DRAFT.  201 

formulae,  properly  transposed,  the  pressure  due  to  any  given  velocity  or  neces- 
sary to  its  creation  may  be  determined.  The  pressure  thus  determined  is 
properly  that  which  it  is  the  purpose  of  a  fan,  employed  as  a  device  for  moving 
air,  to  create.  What  reactionary  pressure  this  velocity  may  produce  as  the  air 
escapes  from  the  fan  is,  therefore,  in  this  connection,  a  matter  of  secondary 
importance. 

The  velocity  of  the  fan  tips  or  circumference  of  the  fan  wheel  which  is  neces- 
sary to  produce  a  given  velocity  of  flow  through  a  properly  shaped  outlet  within 
the  capacity  of  the  fan  is  substantially  equal  to  that  velocity  of  flow.  If,  there- 
fore, the  peripheral  velocity  of  a  given  fan  is  that  indicated  by  any  given 
quantity  in  column  3  of  Table  No.  91,  the  resulting  pressure  for  the  production 
of  velocity  through  an  outlet  of  proper  size  and  shape  will  be  practically  that 
which  corresponds  to  the  given  velocity  in  the  table. 

From  the  basis  formula  employed  in  the  calculation  of  this  table,  as  well  as 
from  preceding  discussions,  it  is  evident  that  the  pressure  created  by  a  given 
fan  varies  as  the  square  of  its  speed.  The  volume  of  air  delivered  is,  however, 
practically  constant  per  revolution,  and  therefore  is  directly  proportional  to  the 
speed.  The  volume  discharged  under  given  pressure  and  velocity  through  an 
opening  of  given  effective  area  is  presented  in  Table  No.  91.  From  this  the 
total  volume  of  air  discharged  may  be  calculated  for  any  other  opening  whose 
area  is  known. 

The  work  done  by  a  fan  in  moving  air  is  represented  by  the  distance  through 
which  the  total  pressure  is  exerted  in  a  given  time.  As  ordinarily  expressed  in 
foot-pounds,  the  work  per  second  would,  therefore,  be  the  product  of  the  velocity 
of  the  air  in  feet  per  second,  the  pressure  in  pounds  per  square  foot,  and  the 
effective  area  in  square  feet  over  which  the  pressure  is  exerted.  If  W  repre- 
sents the  work  done,/  the  pressure,  a  the  area  and  v  the  velocity,  the  expres- 
sion for  the  work  becomes  — 

W  =par 
But  it  has  previously  been  shown  that  — 


-J 


hence — 

dv2 

Therefore,  the  value  of  W  becomes  — 


202  MECHANICAL   DRAFT. 

from  which  it  is  evident  that  the  work  done  varies  as  the  cube  of  the  velocity  ; 
that  is,  as  the  cube  of  the  revolutions  of  the  fan.  The  reason  is  evident  in  the 
fact  that  the  pressure  increases  as  the  square  of  the  velocity,  while  the  velocity 
itself  coincidently  increases ;  hence  the  product  of  these  two  factors  of  the 
power  required  is  indicated  by  the  cube  of,  the  velocity. 

As  one  horse-power  is  equivalent  to  33,000  foot-pounds  of  work  per  minute, 
the  horse-power  for  a  given  area  of  discharge  in  square  inches,  when  the  value 
of  g  is  taken  as  32.2,  may  be  expressed  by  — 


144       33>°°°  X  64-4 

dav* 
~  5,100,480 

The  horse-power  required  to  move  air  under  various  pressures  and  velocities 
has  already  been  calculated  and  presented  in  Table  No.  91.  As  applied  to  the 
case  of  a  fan,  the  quantities  there  given,  being  for  one  square  inch  of  effective 
area,  must  be  multiplied  by  the  total  effective  area  through  which  the  fan  dis- 
charges and  also  by  a  coefficient  which  is  the  reciprocal  of  the  efficiency.  This 
coefficient,  which  under  favorable  working  conditions  may  be  indicated  by  an 
efficiency  as  high  as  75  per  cent,  must  of  course  be  dependent  upon  the  circum- 
stances under  which  the  particular  fan  is  to  be  operated.  But  under  no  condi- 
tions can  any  device  move  air  with  the  proportionate  expenditure  of  power 
indicated  in  Table  No.  91  ;  for,  as  already  shown,  this  value  does  not  include 
the  losses  due  to  frictional  resistances  of  the  machine  itself  and  of  the  air  in  its 
movement  through  the  machine  and  connecting  outlet  or  pipe. 

As  the  weight  of  the  air  is  dependent  upon  its  temperature  and  the  baromet- 
ric and  hygrometric  conditions,  it  is  evident  that  the  pressure  exerted  and  the 
power  required  by  a  given  fan  may  vary  greatly  even  at  constant  speed.  Under 
ordinary  conditions,  however,  changes  in  the  height  of  the  barometer  and  the 
humidity  of  the  atmosphere  have  no  appreciable  effect  upon  the  pressure  and 
power.  But  the  density  varies  inversely  as  the  absolute  temperature,  and, 
therefore,  should  enter  as  a  factor  even  in  calculations  with  reference  to  air  at 
or  about  ordinary  atmospheric  temperatures,  and  must  be  taken  into  account 
when  heated  air  or  gases  are  handled.  The  importance  of  this  statement  has 
already  been  shown  in  the  discussion  of  Table  No.  93,  and  will  be  further  con- 
sidered in  connection  with  Table  No.  113. 

By  means  of  the  curves  in  Fig.  13  are  graphically  illustrated  the  theoretical 
relations  between  the  revolutions  at  which  a  fan  is  run  and  the  volume  which  is 
discharged,  the  pressure  which  is  created  and  the  horse-power  which  is  required. 


MECHANICAL    DRAFT, 


203 


These  curves  are  based  upon  the  facts  that  the  volume  varies  directly  as  the 
speed,  the  pressure  as  the  square  and  the  horse-power  as  the  cube  of  the  speed. 
Thus  it  is  shown  by  the  curves  that  if  the  speed  is  doubled  the  volume  is  also 


0)13 


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Relative    Revolutions     of     Fan 

FIG.  13.     THEORETICAL  RELATIONS  BETWEEN  THE  REVOLUTIONS  OF  A  FAN,  THE  VOLUME 
DISCHARGED,  THE  PRESSURE  CREATED  AND  THE  HORSE-POWER  REQUIRED. 

doubled,  the  pressure  is  increased  four  times  and  the  horse-power  becomes 
eight  times  greater.  The  tremendous  power  expenditure  which  is  required  for 
even  a  moderate  increase  of  speed  of  the  fan  is  thus  rendered  very  distinctly 
evident. 


204  MECHANICAL    DRAFT. 

In  selecting  a  fan,  the  facts  presented  in  Fig.  13  should  be  borne  in  mind.  It 
appears  to  be  so  simple  to  secure  increased  volume  by  running  a  given  fan  at 
higher  speed  that  the  influence  upon  the  power  required  is  frequently  over- 
looked. If  the  necessary  amount  of  power  is  actually  furnished,  its  expenditure 
will  entail  great  loss  in  efficiency  as  compared  with  that  required  to  operate  a 
fan  properly  proportioned  to  the  work. 

Design  of  Fans.  —  In  the  design  of  a  wheel  to  meet  given  requirements  it  is 
necessary  to  make  its  peripheral  speed  such  as  to  create  the  desired  pressure, 
and  then  so  proportion  its  width  as  to  provide  for  the  required  air  volume. 
Evidently,  the  velocity  and  corresponding  pressure  may  be  obtained  either  with 
a  small  wheel  running  at  high  speed  or  a  large  wheel  running  at  slow  speed. 
But  if  the  diameter  of  the  wheel  be  taken  too  small,  it  may  be  impossible  to 
adopt  a  width,  within  reasonable  limits,  which  will  permit  of  the  passage  of  the 
necessary  amount  of  air  under  the  desired  pressure.  Under  this  condition  it 
will  be  necessary  to  run  the  fan  at  higher  speed  in  order  to  obtain  the  desired 
volume.  But  this  results  in  raising  the  pressure  above  that  desired,  and  in 
unnecessarily  increasing  the  power  required.  On  the  other  hand,  if  the  wheel 
be  made  of  excessive  diameter  it  will  become  almost  impracticable  on  account 
of  its  narrowness.  Between  these  two  extremes  a  diameter  must  be  intelligently 
adopted  which  will  give  the  best  proportions.  If  the  fan  is  to  be  driven  by 
a  direct-connected  engine,  as  is  the  common  practice  in  mechanical  draft,  it  may 
be  further  necessary  to  so  adapt  the  number  of  revolutions,  and  consequently 
the  peripheral  velocity  of  the  wheel,  to  a  given  engine,  that  its  speed  may  not 
be  excessive  and  that  the  proper  amount  of  power  may  be  generated. 

The  actual  volumetric  capacity  of  a  given  fan,  operating  under  practical  con- 
ditions, is  naturally  to  be  sought  as  a  means  of  measuring  it  relatively  to  any 
other  fan.  But  manifestly  such  capacity  is  somewhat  difficult  of  pre-determina- 
tion.  In  the  case  of  a  steam  engine,  its  nominal  rating  —  that  by  which  one 
engine  may  be  measured  relatively  to  another—  is  based  upon  the  diameter  and 
stroke  of  the  cylinder,  the  number  of  revolutions  and  the  mean  effective  pres- 
sure. But  the  power  thus  calculated  by  no  means  represents  the  amount  which 
may  be  delivered  to  a  given  machine,  for  the  sole  purpose  of  operating  which 
the  engine  is  employed.  This  latter  amount  will  be  less  than  that  calculated,  to 
the  extent  that  power  is  absorbed  in  the  internal  friction  of  the  engine  and 
by  the  intermediate  mechanism  of  transmission.  So  in  the  case  of  a  fan  wheel, 
its  theoretical  volumetric  capacity  will  depend  upon  its  dimensions  and  the 
speed  at  which  it  is  operated.  But  in  practice  the  actual  amount  of  air  delivered 
will  also  be  largely  dependent  upon  the  fact  of  the  wheel  being  encased,  the 
character  and  dimensions  of  the  case,  and  the  size  and  resistances  of  the  pas- 


MECHANICAL    DRAFT. 


205 


sages  through  which  the  air  is  conducted.  The  equivalent  of  such  resistances 
is  in  boiler  practice  usually  represented  by  the  grates,  the  fuel,  tubes,  etc.,  and 
may  evidently  be  so  great  at  times  as  to  very  seriously  reduce  the  theoretical  air 
discharge  of  the  fan. 

Evidently,  it  is  improper  to  compare  fans  when  operating  under  such  condi- 
tions that  these  resistances  cannot  be  definitely  determined.  The  simplest  and 
most  natural  condition  is  that  in  which  the  fan  is  operated  without  other  resist- 
ance than  that  of  the  case;  that  is,  with  open  inlet  and  outlet.  But  for  proper 
comparison  of  different  fans,  the  areas  through  which  the  air  is  discharged 
should  bear  some  constant  relation  to  the  dimensions  of  the  wheels  themselves. 

It  has  been  determined  experimentally  that  a  peripheral  discharge  fan,  if 
enclosed  in  a  case,  has  the  ability,  if  driven  to  a  certain  speed,  to  maintain  the 
pressure  corresponding  to  its  tip  velocity  over  an  effective  area  which  is  usually 
denominated  the  "square  inches  of  blast."  This  area  is  the  limit  of  its  capacity 
to  maintain  the  given  pressure.  If  it  be  increased  the  pressure  will  be  reduced, 
but  if  decreased  the  pressure  will  remain  the  same.  As  fan  housings  are 
usually  constructed,  this  area  is  considerably  less  than  that  of  either  the  regular 
inlet  or  outlet.  It,  therefore,  becomes  necessary,  in  comparing  fans  upon  this 
basis,  to  provide  either  the  inlet  or  the  outlet  with  a  special  temporary  orifice 
of  the  requisite  area  and  proper  shape,  and  make  proper  correction  for  the 
contracted  vein.  The  fan  is  thus,  in  a  sense,  placed  in  a  condition  of  restric- 
tion of  discharge,  which  it  approaches  in  practice  only  in  so  far  as  the  resist- 
ances of  pipes,  passages  and  material  through  which  the  air  must  pass  have 
the  effect  of  reducing  the  free  inlet  or  outlet  of  the  fan. 

The  square  inches  of  blast,  or,  as  it  may  be  termed,  the  capacity  area  of  a 
cased  fan,  may  be  approximately  expressed  by  the  empirical  formula, — 

DW 

Capacity  area  = 

In  which  D  =  diameter  of  fan  wheel,  in  inches. 

W  =  width  of  fan  wheel  at  circumference,  in  inches. 

x  =  a  constant  dependent  upon  the  type  of  fan  and  casing. 

The  value  of  x  has  been  very  carefully  determined  by  this  Company  for 
different  types  of  fans,  but  these  values  must  be  applied  with  great  discretion, 
acquired  through  experience  and  a  thorough  knowledge  of  all  the  conditions 
liable  to  affect  the  fan  in  operation.  An  approximate  value  of  x  for  general 
practice  is  not  far  from  3,  but  this  is  to  be  used  only  to  determine  the  capacity 
area  over  which  the  given  pressure  may  be  maintained.  This  is  not  a  measure 
of  the  area  of  the  casing  outlet,  which  is  always  larger  than  the  square  inches 


2o6  MECHANICAL   DRAFT. 

of  blast.  As  a  consequence,  the  pressure  is  lower  and  the  volume  discharged 
is  somewhat  greater  than  would  result  through  an  outlet  having  the  square 
inches  of  blast  for  its  area.  But  the  maximum  pressure  may  be  realized  when 
the  sum  of  resistances  is  equivalent  to  a  reduction  of  effective  outlet  area  to 
that  of  square  inches  of  blast.  The  volume  of  air  which  under  the  given  pres- 
sure will  flow  through  the  given  capacity  area,  and  hence  the  volumetric  capacity 
of  the  fan  under  the  given  conditions,  may  be  determined  from  Table  No.  91. 
In  a  similar  manner  the  horse-power  may  be  ascertained,  the  proper  efficiency 
coefficient  being  applied. 

Both  the  volume  and  the  power  required  will  evidently  increase  with  the  area 
of  the  outlet,  being  greater  with  the  normal  outlet  than  with  that  representing 
the  capacity  area.  But  this  increase  will  not  be  proportional  to  the  area,  for  the 
pressure  and  consequently  the  velocity  will  be  lower  with  the  larger  area.  The 
greatest  delivery  of  air  and  the  largest  consumption  of  power  will  occur  when 
the  casing  is  entirely  removed  and  the  fan  left  free  to  discharge  entirely  around 
its  periphery. 

Although  the  theoretical  considerations  which  govern  the  design  of  fans  have 
here  been  given,  the  conditions  which  exist  in  any  given  case  must  enter  into 
any  decision  as  to  the  practical  dimensions  to  be  given  the  fan. 

If  volume  alone,  regar.dless  of  pressure,  is  the  requisite,  the  larger  the  fan,  the 
less  the  power  required.  There  is  a  strong  temptation,  however,  for  a  purchaser 
to  buy  a  smaller  fan  and  run  it  at  higher  speed ;  for  he  sees  only  the  first  cost 
and  does  not  realize  the  entailed  expenditure  for  extra  power.  If  possible,  a 
fan  should  never  be  made  so  small  that  it  is  necessary  to  run  it  above  the 
required  pressure  in  order  to  deliver  the  necessary  volume.  To  double  the  vol- 
ume under  such  circumstances  requires  eight  times  the  power;  three  times  the 
volume  demands  twenty-seven  times  the  power. 

For  certain  purposes,  such  as  the  blowing  of  cupola  furnaces,  a  comparatively 
small  volume  of  air  is  required,  but  under  high  pressure.  In  steam-boiler  prac- 
tice the  volume  is  relatively  greater,  and  the  pressure  less ;  the  general  range  of 
pressures  required  has  already  been  shown.  The  former  wheel  requires  to  be 
narrow  at  the  circumference,  thus  providing  for  the  escape  of  only  a  small 
amount  of  air.  When  a  fan  is  employed  for  exhausting  hot  air  or  gases,  the 
speed  required  to  maintain  a  given  pressure  difference  is  evidently  greater 
than  that  necessary  when  cold  air  is  handled,  the  difference  being  due  to,  and 
inversely  proportional  to,  the  absolute  temperature. 

From  the  last  formula  it  is  evident  that  if  the  diameter  of  the  wheel  be  known, 
as  determined  by  the  previous  considerations,  its  width  may  be  determined  for 
any  given  capacity  area  when  the  value  of  x  is  ascertained.  If,  furthermore, 


MECHANICAL    DRAFT.  207 

the  pressure  at  which  it  is  to  operate  be  pre-determined,  the  number  of  revolu- 
tions may  be  readily  ascertained  by  means  of  Table  No.  91  ;  for  the  tip  speed 
must  be  equal  to  the  velocity  there  given  as  corresponding  to  the  stated  pres- 
sure. From  the  same  table  the  volume  of  air  delivered  and  the  theoretical 
horse-power  required  for  the  given  capacity  area  may  be  determined.  How 
much  greater  than  these  amounts  the  actual  results  will  show  must  of  course 
depend  upon  the  circumstances,  particularly  the  area  of  discharge  existing  in 
each  case.  For  the  more  ready  determination  of  the  number  of  revolutions  of 
any  given  size  of  fan  which  are  necessary  to  produce  a  given  pressure  over  the 
capacity  area  of  the  wheel,  Table  No.  112  has  been  prepared.  This  is  self- 
explanatory.  Being  based  on  the  values  in  Table  No.  91,  these  results  apply 
only  to  a  temperature  of  50°  Fahr.  For  other  temperatures  due  correction 
must  be  made. 

In  Table  No.  93  has  already  been  indicated  the  effect  of  changes  in  tempera- 
ture ;  but,  as  there  presented,  they  apply  primarily  to  the  effect  upon  the  pres- 
sures required  to  produce  given  velocities.  As  it  is  the  province  of  a  fan  to 
operate  at  certain  velocities  in  order  to  produce  desired  pressures,  the  important 
temperature  factors  are  presented  in  Table  No.  113  in  somewhat  different  form. 
The  temperatures  in  the  table  are  in  degrees  Fahrenheit  above  zero.  These 
results  are  also  graphically  presented  in  Fig.  14,  the  .curves  representing  the 
values  in  the  columns  as  designated. 

The  basis  upon  which  the  various  relations  have  been  calculated  is  as  follows, 
the  atmospheric  temperature  being  taken  at  50°  Fahr.  and  absolute  zero  at  461° 
below  zero,  Fahr. :  — 

Column  2.  The  volume  of  the  same  weight  of  air  is  directly  proportional  to 
its  absolute  temperature. 

Column  3.  The  weight  of  the  same  volume  of  air  is  inversely  proportional 
to  its  absolute  temperature. 

Column  4.  The  speed  of  the  same  fan  necessary  to  handle  the  same  weight 
of  air  is  directly  proportional  to  its  absolute  temperature. 

Column  5.  The  pressure  difference  due  to  the  speed  of  the  same  fan  neces- 
sary to  handle  the  same  weight  of  air  is  directly  proportional  to  the  square  of 
the  speed  and  inversely  proportional  to  the  absolute  temperature  of  the  air. 

Column  6.  The  pressure  difference  due  to  the  same  speed  of  fan  handling 
the  same  volume  of  air  is  inversely  proportional  to  its  absolute  temperature. 

Column  7.  The  speed  necessary  to  produce  the  same  pressure  difference  is 
directly  proportional  to  the  square  root  of  the  absolute  temperature. 

Column  8.  The  power  required  for  the  same  speed  and  volume  is  inversely 
proportional  to  the  absolute  temperature. 


208 


MECHANICAL   DRAFT. 


Table  No.   112.  —  Revolutions  of  Fan  of  Given  Diameter  Necessary  to   Maintain 
a  Given  Pressure  over  an  Area  which  is  within  the 
Capacity  of  the  Fan. 


PRESSURE,  ix  OUNCES  I>ER  SQUARE  INCH. 


Fan  Wheel, 
in  Feet. 

X 

X 

N 

% 

X 

X 

7A 

i 

I# 

«# 

1% 

fjt 

»* 

I 

582 

823 

1,007 

1,163 

1,300 

1,423 

1,537 

t,643 

1,742 

1,836 

1,925 

2,010 

2,170 

*X 

466 

658 

806 

93° 

1,040 

1>139 

1,230 

t.3'4 

1-394 

1,469 

1,540 

1,  608 

i,736 

»# 

388 

549 

672 

'775 

867 

949 

1,025 

i,095 

1,162 

1,224 

1,284 

1,340 

i,447 

'#    333 

470 

576 

665 

743 

813 

878 

938 

996 

1,049 

1,100 

1,149 

1,240 

2      !   291 

411 

504 

582 

650 

712 

769 

822 

871 

918 

963 

1,005 

1,085 

2# 

259 

366 

448 

5i7 

578 

633 

683 

73° 

774 

816 

856 

893 

964 

2^ 

233 

329 

403 

465 

520 

570 

615 

657 

697 

734 

770 

804 

868 

2%    \   212 

300 

366 

423 

473 

5i8 

559 

597 

634 

668 

700 

73i 

789 

3    i  194 

274 

336 

388 

433 

475 

5i3 

548 

581 

612 

642 

670 

723 

3/2 

1  66 

235 

288 

332 

372 

407 

439 

469 

498 

525 

55° 

574 

620 

4 

146 

206 

252 

291 

325 

356 

384 

411 

436 

459 

481 

502 

543 

4# 

129 

183 

224 

258 

289 

316 

342 

365 

387 

408 

428 

447 

482 

5 

116 

164 

202 

232 

260 

285 

3°8 

329 

349 

367 

385 

402 

434 

& 

1  06 

149 

183 

211 

236 

259 

280 

299 

3'7 

334 

35° 

366 

395 

6 

97 

137 

1  68 

194 

217 

238 

256 

274 

290 

306 

321 

335 

362 

6^ 

90 

126 

J55 

179 

200 

219 

236 

253 

268 

282 

296 

309 

334 

7 

83 

117 

144 

1  66 

1  86 

203 

220 

235 

249 

26.- 

275 

287 

310 

7/2 

78 

no 

'35 

155 

173 

190 

204 

219 

232 

245 

257 

268 

289 

8 

73 

i°3 

126 

146 

163 

178 

192 

205 

218 

230 

241 

25J 

271 

8/2 

69 

97 

119 

137 

'53 

167 

181 

194 

205 

216 

226 

236 

255 

9 

65 

92 

112 

129 

144 

158 

171 

183 

194 

204 

214 

223 

241 

9X 

61 

87 

106 

123 

J37 

149 

162 

173 

183 

J93 

203 

212 

228 

10 

58 

82 

IOI 

116 

130 

142 

154 

164 

'74 

184 

193 

201 

217 

it 

53 

75 

92 

1  06 

118 

129 

140 

150 

158 

167 

J75 

I83 

197 

12 

49 

69 

84 

97 

1  08 

119 

128 

137 

M5 

153 

1  60 

1  68 

181 

13 

45 

63 

78 

90 

100 

no 

116 

126 

13° 

141 

148 

155 

167 

14 

42 

59 

72 

83 

93 

102 

no 

"7 

124 

'31 

138 

144 

155 

>5 

39 

55 

67 

78 

87 

95 

102 

I  10 

116 

I  22 

128 

J34 

J45 

MECHANICAL   DRAFT. 


209 


Table  No.  112.  —  Revolutions  of  Fan  of  Given  Diameter  Necessary  to  Maintain 
a  Given  Pressure  over  an  Area  which  is  within  the  Capacity 
of  the  Fan.  —  Concluded. 


Diameter  of 
Fan  Wheel, 

PRESSURE,  IN  OUNCES  PER  SQUARE  INCH. 

1 

in  Feet. 

• 

2 

*# 

3 

3/2 

4 

-^   5 

5% 

6 

6/2 

7 

7/2 

8 

( 

2,3!9 

2,590 

2,834 

3.058 

3,265 

3,460 

3,643 

3,8i7 

3,992 

4,141 

4,293 

4,439 

4,580 

t* 

1.855 

2,072 

2,267 

2,446 

2,6l2 

2,768 

2,915 

3,054 

3,186 

3,313 

3.434 

3.551 

3.664 

1/2 

1,546 

1.727 

1,889 

2,039 

2,178 

2,307 

2,429 

2,545 

2,655 

2,761 

2,862 

2,960 

3,053 

'# 

'.325 

1,480 

1,619 

1,747 

1,866 

i,977 

2,082 

2,171 

2,276 

2,366 

2,453 

2,536 

2,617 

2 

1.159 

1.295 

i,4i7 

',529 

J>633 

1,73° 

1,822 

1,909 

1,996 

2,070 

2,146 

2,219 

2,289 

2# 

1,030 

1,151 

1,259 

r'359 

1,451 

i,538 

1,619 

1,696 

1,770 

1,840 

1,908 

L973 

2.035 

** 

928 

1,036 

1,134 

1,223 

1,306 

t,384 

i,457 

1,527 

'.593 

1,656 

i,7i7 

1,776 

1,832 

*x 

843 

942 

1,030 

1,112 

1,188 

1,258 

L325 

1,388 

1,448 

1,506 

1,561 

1,614 

1,665 

3 

773 

863 

945 

1,019 

1,089 

M53 

1,215 

1,272 

1,328 

1,380 

i,43i 

1,480 

f,527 

3/2 

662 

740 

810 

874 

933 

989 

1,041 

i,  086 

1,138 

1,183 

1,226 

1,268 

1,308 

4 

580 

647 

708 

764 

816 

865 

911 

954 

998 

!,035 

1,073 

1,110 

i,  145 

4/2 

5*5 

575 

630 

679 

726 

769 

810 

848 

885 

920 

954 

986 

1,018 

5 

464 

5i8 

567 

612 

653 

692 

729 

763 

796 

828 

859 

888 

916 

5^ 

422 

47i 

5i5 

556 

594 

629 

662 

694 

724 

753 

78i 

807 

833 

6 

386 

432 

472 

510 

545 

577 

607 

636 

664 

690 

716 

740 

763 

6# 

357 

398 

436 

470 

502 

S32 

56i 

587 

613 

637 

661 

683 

705 

7 

33i 

370 

405 

437 

466 

494 

520 

543 

569 

592 

613 

634 

654 

7/2 

3°9 

345 

378 

408 

435 

461 

486 

509 

53i 

SS2 

572 

592 

611 

8 

290 

324 

354 

382 

408 

432 

455 

477 

499 

518 

537 

555 

572 

sx 

273 

3°5 

333 

33i 

354 

375 

429 

449 

469 

487 

505 

522 

539 

9 

258 

288 

3'5 

340 

363 

384 

405 

424 

443 

460 

477 

493 

509 

9X 

244 

273 

298 

322 

344 

364 

384 

402 

419 

436 

452 

467 

482 

10 

232 

259 

283 

306 

327 

346 

364 

382 

398 

414 

429 

444 

458 

ii 

211 

235 

258 

278 

247 

3'5 

33i 

347 

362 

376 

390 

404 

4,6 

12 

193 

216 

236 

255 

272 

288 

3°4 

3i8 

332 

345 

358 

3/0 

382 

13 

I78 

199 

218 

235 

251 

266 

280 

294 

306 

3'9 

33° 

341 

352 

M 

I65 

185 

202 

218 

233 

247 

260 

271 

284 

296 

3°7 

3>7 

327 

15 

'55 

173 

189 

204 

218 

231 

243 

254 

266 

276 

286 

291 

305 

MECHANICAL    DRAFT. 


Table  No.   113.  —  Relation  of  Volume,  Weight  and  Pressure  of  Air  and  Speed 
Power  of  a  Fan  with  Air  at  Different  Temperatures. 


md 


Temper- 
ature, 
Degrees 

rate. 

4 

Volume 
for  Same 
Weight. 

2 

Weight 
for  Same 
Volume. 

3 

Speed  of 
Fan 
to  Handle 
Same 
Weight. 

4 

Pressure 
Difference 
due  to 
Speed 
Necessary 
to  Handle 
Same 
Weight. 

5 

Pressurfe 
Difference 
for  Same 
Speed  and 
Volume. 

6 

Speed  to 
Produce 
Same 
Pressure 
Difference. 

7 

Power 
for  Same 
Speed  and 
Volume. 

8 

Power 

for  Speed 
Necessary 
to  Handle 
Same 
Weight. 

9 

Power 

Necessary 
to  Handle 
Same 
Weight  at 
Same 
Pressure 
with  a 
Properly 
Propor- 
tionedFan. 

300 

0.96 

1.04 

0.96 

0.96 

1.04 

0.98 

1.04 

0.92 

0.96 

40 

0.98 

1.02 

0.08 

0.98 

I.  O2 

0.99 

1.02 

0.96 

0.98 

5° 

1.  00 

1.  00 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

60 

1.  02 

0.98 

1.02 

1.02 

0.98 

1.01 

0.98 

1.04 

1.02 

70 

1.04 

0.96 

1.04 

I.O4 

0.96 

1.02 

0.96 

1.  08 

1.04 

80 

1.  06 

0.94 

1.  06 

1.  06 

0.94 

1.03 

0.94 

1.  12 

.06 

90 

1.  08 

o-93 

1.  08 

1.  08 

o-93 

1.04 

Q-93 

I.I7 

.08 

IOO 

I.IO 

0.91 

I.IO 

I.IO 

0.91 

1.05 

0.91 

1.  21 

.10 

I25 

«-«3 

0.87 

1.15 

1-1S 

0.87 

1.07 

0.87 

I.32 

•15 

'5° 

1.20 

0.84 

1.20 

1.20 

0.84 

1.09 

0.84 

i-43 

.20 

17S 

1.24 

0.8  1 

1.24 

1.24 

0.8  1 

I.  II 

0.8  1 

'•55 

.24 

200 

1.29 

0.78 

1.29 

1.29 

0.78 

I.I4 

0.78 

1.67 

.29 

225 

i-34 

0-75 

i-34 

i-34 

0.75 

1.16 

o-75 

i.  80 

•34 

250 

'•39 

0.72 

i-39 

i-39 

0.72 

1.18 

0.72 

i-93 

•39 

275 

1.44 

0.69 

1.44 

1.44 

0.69 

1.20 

0.69 

2.07 

•44 

300 

i-49 

0.67 

1.49 

1.49 

0.67 

1.22 

0.67 

2.22 

•49 

J2S 

i-54 

0.65 

i-54 

i-54 

0.65 

1.24 

0.65 

2.36 

•54 

35° 

1.59 

0.63 

1.59 

1.59 

0.63 

1.26 

0.63 

2-51 

•59 

375 

1-63 

0.61 

1.63 

1.63 

0.61 

1.28 

0.61 

2.68 

1.63 

400 

1.68 

0.59 

1.68 

1.68 

0.59 

I.30 

o-59 

2.84 

1.68 

425 

i-73 

0.58 

!-73 

J-73 

0.58 

I.32 

•    0.58 

3.01 

i-73 

450 

1.78 

0.56 

1.78 

1.78 

0.56 

i-34 

0.56 

3.18 

1.78 

475 

1.83 

o-55 

1.83 

1.83 

o-55 

'•35 

o-55 

3-35 

1-83 

500 

1.88 

o-53 

1.88 

1.88 

o-53 

i-37 

o-53 

3-56 

1.88 

525 

i-93 

0.52 

i-93 

i-93 

0.52 

J-39 

0.52 

3-71 

i-93 

55° 

1.98 

0.51 

1.98 

1.98 

0.51 

1.41 

0.51 

3-92 

1.98 

575 

2.03 

0.49 

2.03 

2.03 

o-49 

i-43 

0.49 

4.12 

2.03 

600 

2.08 

0.48 

2.08 

2.08 

0.48 

1.44 

0.48 

4-32 

2.08 

625 

2.13 

0.47 

2.13 

2-13 

0.47 

1.46 

0.47 

4-54 

2.13 

650 

2.18 

0.46 

2.18 

2.18 

0.46 

1.48 

0.46 

4-75 

2.18 

675 

2.22 

0-45 

2.22 

2.22 

0.45 

1.49 

0.45 

4-93 

2.22 

700 

2.27 

0.44 

2.27 

2.27 

0.44 

I-5I 

0.44 

5.16 

2.27 

725 

2.32 

0-43 

2.32 

2.32 

o-43 

J-52 

0-43 

5-39 

2.32 

75° 

2-37 

0.42 

2-37 

2-37 

0.42 

F.-S4 

0.42 

5.62 

2-37 

775 

2.42 

0.41 

2.42 

2.42 

0.4  1 

1.56 

0.41 

5.86 

2.42 

800 

2.47 

0.40 

2.47 

2.47 

0.40 

i-57 

0.40 

6.10 

2.47 

MECHANICAL   DRAFT.  211 

Column  9.  The  power  required  to  operate  the  same  fan  at  the  speed  neces- 
sary to  handle  the  same  weight  of  air  is  directly  proportional  to  the  cube  of  the 
speed  and  inversely  proportional  to  the  absolute  temperature. 

Column  10.  The  power  necessary  to  handle  the  same  weight  of  air  at  the 
same  pressure  difference  by  means  of  a  properly  proportioned  fan  is  directly 
proportional  to  the  speed  and  to  the  area  required  for  the  passage  of  the  given 
weight  corresponding  to  the  speed.  That  is,  it  is  directly  proportional  to  the 
absolute  temperature. 

The  conditions  indicated  in  column  10  are  substantially  those  usually  pre- 
sented in  induced-draft  practice,  wherein  it  is  generally  necessary  to  move  the 
same  weight  of  air  under  the  same  pressure,  but  at  a  higher  temperature  than 
would  be  required  in  the  case  of  air  handled  for  forced  draft.  From  column  4 
it  is  evident  that  if  with  the  same  fan  it  be  attempted  to  handle  the  same  weight 
at  increased  temperature,  the  speed  must  also  be  increased;  and  it  is  further 
shown,  per  column  9,  that  the  power  required  under  these  circumstances  rises 
very  rapidly  with  the  temperatures.  But  a  fan  so  designed  as  to  move  the  same 
weight  without  exceeding  the  pressure  created  at  the  lower  pressure  will  require 
far  less  power. 

Thus  suppose  that  a  fan  is  to  be  designed  to  handle  air  or  gases  (they  both 
being  here  considered,  for  simplicity,  as  of  the  same  specific  gravity)  at  a  tem- 
perature of  300°.  At  this  temperature  the  speed  necessary  to  maintain  the 
same  pressure  is  relatively  1.22,  per  column  7.  But  through  a  given  area  of 
opening,  such  as  would  represent  the  capacity  area  of  a  proper  fan  for  the  lower 
temperature,  the  actual  weight  moved  would  be  the  product  of  the  velocity  and 
the  density;  that  is,  only  1.22  x  0.67  =  0.82  at  300°.  In  order  to  maintain  the 
weight  at  unity,  the  area  of  discharge  for  the  given  velocity  must  therefore  be 

increased  until  it  becomes  —~  =1.22.     So  that  the  power  actually  expended 

O.o2 

in  moving  the  same  weight  with  a  properly  designed  fan  would  for  the  same 
pressure  be,  at  300°,  the  combined  product  of  the  unit  pressure,  the  area  and 
the  velocity;  that  is,  i  X  1.22  X  1.22  =  1.49  times  that  required  for  a  fan  prop- 
erly designed  to  maintain  the  same  pressure  and  handle  the  weight  of  air  at  50°. 
If  the  same  diameter  of  fan  be  employed  under  each  condition,  and  the 
greater  volume,  in  the  case  of  the  higher  temperature,  be  provided  for  by  widen- 
ing the  fan,  the  power  required  per  revolution  will  be  in  the  relation  of  i  at 
50°  to  1.22  at  300°  temperature.  For,  as  already  shown,  the  total  power  re- 
quired at  the  latter  temperature  is  1.49,  while  the  speed  is  1.22,  both  relatively  to 

the  conditions  at  50°;   hence  the  power  per  revolution  is  i^2  ==  1.22.      From 


212 

6 


MECHANICAL   DRAFT. 


z 


Is 


100       200       300      400       500       600       JOO       800       900 

Temperatures    in    degrees    Fahrenheit    above    zero 

FIG.  14.     RELATION  OF  VOLUME,  WEIGHT  AND  PRESSURE  OF  AIR  AND  SPEED  AND  POWER 
OF  A  FAN  WITH  AIR  AT  .DIFFERENT  TEMPERATURES. 


MECHANICAL    DRAFT.  213 

this  it  is  evident  that  if  in  each  case  the  fan  be  driven  by  a  direct-connected  en- 
gine, whose  revolutions  of  necessity  correspond  to  those  of  the  fan,  the  engine 
will  require  additional  piston  area,  or  mean  effective  pressure,  to  the  extent  of 
only  22  per  cent. 

From  Table  No.  113  it  is  evident  that,  if  a  fan  be  designed  to  handle  a  given 
weight  and  corresponding  volume  of  air  at  50°  under  a  given  pressure  and  with 
the  expenditure  of  a  given  amount  of  power,  the  following  conditions  will  hold, 
if  the  temperature  of  the  air  be  raised  to  300°  for  instance.  If  the  same  weight 
is  to  be  handled,  the  same  fan  will  (per  column  4)  have  to  be  run  at  1.49  times 
the  speed.  At  this  speed  the  pressure  difference  produced  will  (per  column  5)  be 
1.49  times,  and  the  power  expended  (per  column  9)  2.22  times,  that  under 
the  first  condition.  For  the  same  speed  and  volume  the  weight  handled  will 
(per  column  3)  be  0.67,  the  pressure  (per  column  6)  0.67  and  the  power 
(per  column  8)  0.67  of  that  at  a  temperature  of  50°;  and  to  produce  the  same 
pressure  difference  the  fan  will  have  to  run  (per  column  7)  at  1.22  times  the 
speed  required  at  50°.  From  this  it  is  further  evident  that  in  the  attempt  to 
handle,  with  a  given  fan,  the  same  weight  of  air  at  a  higher  temperature  it  is 
necessary  to  increase  the  speed  above  that  required  to  produce  the  same  pres- 
sure difference  ;  and  that  the  power  expended  is  correspondingly  and  unneces- 
sarily increased. 

It  is,  therefore,  obvious  that  the  fan  should  be  designed  to  meet  the  spe- 
cific conditions.  Thus  a  properly  proportioned  fan  will  produce  the  same 
pressure  and  handle  the  same  weight  of  air  at  300°  with  1.49  times  the 
power  (per  column  10)  required  to  obtain  the  same  results  with  the  air  at 
50°;  that  is,  with——  =0.55  =  only  70  per  cent  of  that  which  would  have 

been  necessary  had  the  same  fan  designed  for  50°  been  used  in  both  cases. 

These  are  purely  theoretical  relations,  but  they  hold  substantially  under  prac- 
tical conditions.  The  leakage  of  air  through  boiler  settings  and  the  decrease 
of  efficiency  through  losses  in  power  transmission,  although  they  affect,  yet  do 
not  properly  enter  into  these  relations,  but  must  be  provided  for  by  additional 
capacity  in  fan  and  engine.  Similarly,  the  increase  of  volume  due  to  the  prod- 
ucts of  combustion  may  be  provided  for. 

Methods  of  Application.  —  The  methods  of  application  of  mechanical  draft 
may  be  broadly  classified  under  two  heads,  —  the  plenum  and  the  vacuum 
methods.  Although  both  were  experimented  upon  by  Stevens  in  1827  and  in 
the  succeeding  years,  yet  the  former  remained  for  a  long  period  practically  the 
only  form  in  which  mechanical  draft  was  applied.  As  the  term  implies,  the 
air  under  the  plenum  method  is  forced  through  the  fire;  that  is,  the  pressure 


214  MECHANICAL   DRAFT. 

maintained  below  the  fire  is  greater  than  that  of  the  atmosphere ;  hence  the 
general  term,  "  forced  "  draft. 

Under  the  plenum  method  the  air  may  be  supplied  in  either  of  two  ways. 
First,  by  making  the  ashpit  practically  air  tight,  and  then  forcing  into  it  the  air 
in  sufficient  quantity  and  under  the  requisite  pressure.  Evidently,  the  only 
escape  for  the  air  being  through  the  fuel,  it  must  all  be  utilized  for  the  purposes 
of  combustion.  Second,  by  making  the  fire  room  itself  practically  air  tight  and 
maintaining  therein  the  required  air  pressure  by  means  of  a  fan  of  sufficient 
capacity  to  constantly  make  good  the  amount  of  air  which  under  pressure  passes 
to  the  ashpits  and  thence  through  the  fuel. 

Under  the  vacuum  method  there  is  practically  only  one  means  of  application, 
—  that  by  the  introduction  of  an  exhausting  fan  in  the  place  of  a  chimney. 
This  is  commonly  known  as  the  "induced"  or  "  suction  "  method.  The  fan 
thus  serves  to  maintain  the  vacuum  which  would  exist  if  a  chimney  were  em- 
ployed, and  its  capacity  can  be  made  such  as  to  handle  the  gases  which  result 
from  the  processes  of  combustion.  A  short  and  comparatively  light  stack 
usually  serves  to  carry  these  gases  sufficiently  high  to  permit  of  their  harmless 
escape  to  the  atmosphere. 

Evidently,  the  method  of  application  to  be  adopted  must  depend  upon  the 
circumstances.  It  cannot  be  said  that  under  all  conditions  any  one  of  these 
three  principal  methods,  or  their  numerous  modifications,  is  superior  to  the 
others.  It  is  the  primary  purpose  of  this  book  to  show  the  superior  advantages 
of  mechanical  draft,  and,  secondarily,  to  point  out,  so  far  as  possible,  the  con- 
ditions under  which  any  given  system  may  prove  most  efficient ;  but  it  is  not  the 
purpose  to  advocate  any  one  system,  whether  patented  or  not,  in  preference  to 
the  others.  This  Company  is  interested  in  any  system  that  embodies  the  use  of 
a  fan  ;  for  the  successful  application  of  a  fan  for  such  a  purpose  requires  the 
extended  experience  which  it  has  been  the  privilege  of  this  Company  to  enjoy. 

In  the  succeeding  pages  will  be  presented  detailed  descriptions  of  the  vari- 
ous methods  of  application,  and  of  the  general  advantages  of  mechanical  draft. 

Closed  Ashpit  System.  —  This  system  was  naturally  first  applied  because  of 
its  ready  adaptability  to  existing  conditions.  And  for  the  same  reason  it  has, 
to  a  great  extent,  been  the  system  introduced  wherever,  in  an  existing  plant,  it 
has  been  desired  to  burn  a  cheaper  grade  of  fuel  or  to  add  to  the  steaming 
capacity.  As  most  simply  applied  for  stationary  boiler  work,  the  air  has  been 
introduced  in  the  side  of  the  ashpit  setting  through  a  pipe  from  the  fan.  There 
is  a  tendency,  however,  with  such  a  simple  arrangement,  to  fail  to  properly  dis- 
tribute the  air  in  the  ashpit.  The  result  of  unequal  distribution,  as  may  occur 
with  improper  introduction  of  the  air,  is  a  tendency  to  blow  holes  through  the 


MECHANICAL   DRAFT.  215 

fire  and  overheat  the  grate  bars  wherever  the  draft  is  concentrated.  Of  course, 
the  more  intense  the  draft  the  greater  this  tendency.  This  may  readily  be  over- 
come by  properly  deflecting  the  air  by  means  of  a  Sturtevant  Ashpit  Damper, 
so  as  to  insure  its  thorough  distribution  throughout  the  ashpit  before  it  rises  to 
the  grate. 

A  convenient  form  of  damper,  which  prevents  the  blowing  of  holes  through 
the  fire,  is  illustrated  upon  a  succeeding  page.  It  may  be  placed  either  in  the 
bottom  of  the  ashpit  and  arranged  to  receive  its  air  from  an  underground  duct 
in  front  of  the  boilers ;  or  it  may  be  located  in  the  front  of  the  bridge  wall,  with 
the  hinge  of  the  door  or  damper  above,  so  as  to  deflect  the  air  downward  when 
it  is  opened.  In  either  case  the  damper  can  be  readily  operated  by  a  rod  from 
the  boiler  front.  This  arrangement  necessitates  keeping  the  ashpit  doors  closed. 

In  marine  boilers  with  internal  fire  boxes  such  arrangements  are  usually  in- 
admissible. There  is,  however,  a  method  of  introduction  through  the  back  end 
of  the  ashpit,  the  air  being  conducted  thereto  from  the  back  of  the  boiler  by 
means  of  a  passage  specially  provided  and  extending  through  the  water  back 
and  combustion  chamber.  Ordinarily,  however,  the  air  is  admitted  through  the 
ashpit  doors  or  the  openings  provided  for  them.  This  necessitates  a  removable 
arrangement  so  that  the  ashpits  may  be  cleaned.  Numerous  devices  have  been 
applied  for  this  purpose,  as  is  evident  from  the  illustrations  in  a  succeeding 
chapter. 

The  pressure  within  the  ashpit  and  the  furnace  chamber  causes  all  leakage  to 
be  outward.  The  tendency  is,  therefore,  to  blow  the  ashes  out  of  the  ashpit  and 
the  flame,  smoke  and  fuel  out  of  the  fire  doors,  but  with  slight  effect  in  the  case 
of  stationary  boilers  at  moderate  rates  of  combustion.  In  the  marine  service, 
in  order  to  avoid  inconvenience  from  this  source,  boilers  are  frequently  fitted 
with  false  fronts,  within  which  the  air  pressure  is  maintained.  By  a  proper 
arrangement  of  double  doors  and  dampers  the  disadvantage  and  danger  from 
flame  is  completely  overcome.  The  mere  arrangement  of  dampers  so  connected 
to  the  doors  that  they  close  when  the  fire  doors  are  opened  is  of  great  advan- 
tage. The  false  front  further  presents  an  excellent  yet  simple  means  of  admit- 
ting air  above  the  fuel,  a  feature  which  enters  into  most  arrangements  of  this 
character.  The  conditions  which  have  to  be  met  in  some  cases  of  marine  prac- 
tice are  exemplified  in  Table  No.  114,  in  which  are  presented  the  results  of 
three  tests  of  the  air  pressures  produced  by  Sturtevant  fans  on  U.  S.  S.  Swatara,1 
which  was  equipped  on  the  closed  ashpit  system.  Pressures  are  given  in  inches 
of  water. 


i  Annual  Report  of  the  Chief  of  Bureau  of  Steam  Engineering,  U.  S.  Navy.     1888. 


2l6 


MECHANICAL    DRAFT. 


Although  the  closed  fire-room  system -has  been  far  more  extensively  applied 
in  the  naval  marine  than  has  the  closed  ashpit  system,  it  has  been  due  largely 
to  the  existing  conditions  ;  and  there  can  be  no  doubt  that  of  the  two  the  latter 
arrangement  is  more  satisfactory  when  the  conditions  permit  of  its  introduction. 
The  controlling  conditions  are  well  expressed  in  these  words  of  Engineer-in- 
Chief  George  W.  Melville,  U.  S.  N. : '  "As  between  the  two  methods  of  forced 
draft  in  most  common  use,  that  by  closed  fire  rooms  and  by  closed  ashpits,  I 
am  decidedly  in  favor  of  the  latter  when  it  can  be  applied.  I  make  this  proviso 
for  the  reason  that  some  may  at  once  ask  why,  if  I  am  a  believer  in  ashpit 

Table  No.   114.  —  Air  Pressures  in  Connection  with  Boilers  of  U.  S.  S.  Swatara. 


CONDITIONS. 

' 

* 

3 

Air  pressure  in  conduit    .         .    '     .         .         .         .         . 

2.87 

4.76 

3.48 

Air  pressure  in  ashpit      .        .         .         .         ;         . 

2.08 

4.01 

2-34 

i  86 

•>  61 

Air  pressure  in  furnace    .         .         .         .         .         . 

1.63 

3.18 

!-75 

Air  pressure  in  uptake     .         .'                .         .         . 

0.13 

O.IO 

0.07 

Revolutions  of  fan  per  minute         .         ,        .         ...         . 

438.6 

584.8 

476.6 

forced  draft,  nearly  all  of  our  large  vessels,  recently  designed,  have  forced  draft 
on  the  closed  fire-room  system.  It  is  simply  because,  in  a  war  vessel  with  a 
protective  deck  and  minute  water-tight  sub-divisions,  it  is  extremely  difficult, 
where  there  are  a  number  of  large  boilers,  to  so  arrange  the  blowers  for  closed 

ashpit  forced  draft  as  to  ventilate  the  fire  room  thoroughly The  San 

Francisco,  of  our  navy,  has  ashpit  forced  draft,  and  a!l  who  have  had  experi- 
ence on  her  and  on  other  similar  vessels  speak  in  the  highest  terms  of  praise 
of  the  greater  facility,  convenience  and  comfort  which  attend  this  method. 

"  It  is  to  be  noted,  also,  with  this  method  of  forced  draft,  that  when  there  is 
any  care  at  all  taken  in  the  fire  room  to  keep  the  grate  bars  covered  leaky 
tubes  in  the  combustion  chamber  are  unknown." 

The  absence  of  the  protective  deck,  the  possibility  of  open  fire  rooms  and 
the  greater  space  which  is  usually  available  generally  make  possible  the  closed 
ashpit  system  in  the  merchant  marine,  and  thereby  insure  clean  and  comfortable 
fire  rooms. 


i  Machinery  of  the  New  Vessels  of  the   U.  S.  Navy.     George  W.  Melville.     Transactions 
Society  of  Naval  Architects  and  Marine  Engineers.     1893. 


MECHANICAL    DRAFT. 


217 


This  method  of  application  also  presents  an  opportunity  which  it  shares  with 
the  induced  system  for  utilizing  the  heat  of  the  waste  gases,  which  cannot  be 
attained  with  the  closed  fire-room  system.  The  manner  of  application  will  be 
illustrated  in  a  succeeding  chapter. 

The  practical  results  of  the  introduction  of  the  closed  ashpit  system  in  place 
of  natural  draft  are  clearly  shown  in  the  record  of  four  voyages  of  S.  S.  Dania 
under  each  of  these  conditions,  which  is  here  presented  in  Table  No.  115. 
The  total  consumption  of  coal  per  day  was  reduced  13  per  cent,  while  the  time 
occupied  in  making  the  voyage  was  decreased  nearly  5  per  cent. 

Table  No.   115.— Saving  by  Forced  Draft  on  S.  S.  Dania. 


CONDITIONS. 

Consump- 

Days 
Steaming. 

Knots 
per  Hour. 

Consump-      tion  for  all 
tion  of  Coal      Purposes 
per  Day.         per  Day 

Steaming. 

Natural  draft 

,  four  voyages    

17.00 

7-5° 

9.73         ;      10.70 

Forced  draft, 

four  voyages      .         .         .         .         . 

16.21 

7-58 

7.76                 9.31 

Closed  Fire-Room  System. — This  system  of  mechanical  draft,  in  which  a  ple- 
num condition  is  maintained  in  a  practically  air-tight  fire  room,  is  evidently 
impracticable  for  general  boiler  practice  on  land.  Its  existence  is  in  fact  largely 
due  to  the  exigencies  of  modern  naval  warfare.  It  lends  itself  admirably  to  the 
necessities  of  vessels  in  actual  warfare,  for  it  is  necessary  that  the  openings 
down  to  the  engine  and  boiler  rooms  should  be  kept  as  small  as  possible  ;  and  in 
all  cases  the  machinery  department  would  be  closed  down  and  the  air  supplied 
by  artificial  means  during  an  engagement.  Fans  installed  upon  the  closed  fire- 
room  principle  can  be  easily  arranged  to  ventilate  the  engine  and  fire  rooms 
as  well  as  to  increase  the  combustion  rate.  They  thus  perform  a  double  duty 
and  avoid  the  use  of  a  second  set,  were  this  arrangement  inadmissible. 

The  conditions  existing  in  the  naval  marine  are,  however,  decidedly  different 
from  those  in  the  merchant  service.  In  the  case  of  the  warship  its  maximum 
steaming  capacity  is  seldom  demanded,  and  then  only  for  a  comparatively  short 
time,  as  during  an  engagement.  Consequently  mechanical  draft,  as  applied  to 
vessels  of  this  class,  is  mainly  auxiliary  in  its  character,  but  nevertheless  a 
practical  necessity.  By  its  employment  it  is  possible  to  construct  the  machinery 
within  the  limits  of  space  and  weight  which  are  sufficient  for  ordinary  service, 
while  the  reserve  of  power  is  stored  in  the  light  fans  and  fittings,  instead  of  in 
the  cumbrous  boilers  and  machinery.  In  this  fact  is  summarized  one  of  the 
most  important  advantages  of  mechanical  draft  for  marine  purposes. 


2l8 


MECHANICAL   DRAFT. 


Under  ordinary  cruising  conditions,  when  the  stacks  are  of  moderate  height, 
the  fans  may  not  be  required,  but  they  must  be  of  form,  construction  and 
capacity  sufficient  to  meet  at  an  instant's  notice  the  maximum  demand  that  may 
be  made  upon  them.  For  instance,  a  vessel  of  the  cruiser  type,  which  may  be 
required  in  case  of  necessity  to  develop  9,000  to  10,000  horse-power,  may  at  the 
usual  cruising  speed  of  10  or  12  knots  require  only  1,500  to  2,000  horse-power. 

The  conditions  are  well  exemplified  in  Table  No.  116,'  which  presents  the 
results  of  a  series  of  tests  of  U.  S.  S.  Charleston  at  various  rates  of  speed.  The 
rapidly  increasing  horse-power  with  increased  speed  is  to  be  noted,  as  is  also  the 
far  more  rapid  increase  in  the  power  of  the  blowers,  made  necessary  to  meet 
the  requirements.  The  relation  between  the  actual  efficiency,  as  shown  in  the 
coal  per  indicated  horse-power  and  the  knots  per  ton  of  coal,  is  of  interest  as 
indicating  the  difficulties  in  the  way  of  obtaining  even  a  slight  increase  of  speed 
when  it  is  well  up  to  the  maximum. 

Table  No.  116.  — Results  of  Tests  of  U.  S.  S.  Charleston. 


SPEED  IN  KNOTS. 


I.  H.  P.,  main  engines 
Coal  per  I.  H.  P.  per  hour  . 
I.  H.  P.  of  blowers       . 
Knots  speed  per  ton  of  coal 


13 

>5 

16 

'7 

18 

2,220 

2,820 

3.55° 

4.37° 

5,22O 

6.I2O 

2.2 

2.1 

2.0 

1-9 

2.1 

2-5 

0.0 

2-5 

6.4 

I7.6 

36.8 

69.6 

4-34 

4.00 

3.68 

3-41 

2.84 

2.22 

The  increased  steaming  capacity  per  square  foot  of  grate  and  per  ton  of 
boiler  which  results  from  the  introduction  of  mechanical  draft  is  shown  in  the 
case  of  the  closed  fire-room  system  by  the  results  of  trial  performances  of 
vessels  of  the  British  Navy,  as  presented  in  Table  No.  uy.2  While  in  the  later 
vessels  there  were  improvements  in  the  way  of  more  economical  engines,  yet 
the  greatest  saving  in  weight,  per  indicated  horse-power,  was  brought  about  by 
the  introduction  of  mechanical  draft.  These  tests,  which  were  in  the  early  days 
of  its  introduction  in  the  naval  service,  exerted  a  strong  influence  on  subsequent 
construction.  Said  Mr.  Richard  Sennett,  in  1886  :*  "The  only  system  that  has 


1  Transactions  Society  of  Naval  Architects  and  Marine  Engineers.     1893. 

2  Transactions  Institution  of  Naval  Architects.     London,  1886. 

3  Closed    Stokeholds.      Richard    Sennett.      Transactions    Institution    of    Naval    Architects. 
London,  1886. 


MECHANICAL   DRAFT. 


219 


yet  had  any  extended  practical  trial  is  that  of  closed  stokeholds  worked  under 
air  pressure,  which  was  described  at  considerable  length  by  Mr.  R.  J.  Butler 
in  1883. 

"  Since  that  time  all  ships  built  for  the  Royal  Navy,  and  many  other  vessels, 
have  been  fitted  with  this  system,  and  the  results  obtained  have  been  most 
satisfactory.'' 

Table  No.  117.  —  Results  of  Trial  Performances  of  Similar  Ships  in  British  Navy. 


Steam 

Area 
of  Fire 

I.  H.  P.  per 

Draft. 

SHIP. 

Date. 

Pressure, 
Pounds 
per  Square 

I.  H.  P. 

Weight 
of  Boilers. 

Grate. 

Square 

Inch. 

Tons. 

Square 
Feet. 

Foot  of 
Grate. 

of  Boiler. 

Natural 

f 

Draft, 

|    Inflexible 

1878 

60 

8,483 

756 

829 

IO.2I 

11.22 

Open 

^   Colossus 

1883 

64 

7.492 

594 

645 

11.62 

I2.6l 

Fire 

1   Phaeton  . 

1884 

90 

5,588 

462 

546 

10.23 

12.1 

Rooms. 

I 

Forced 

Draft, 

f  Howe      . 

1885 

90 

11,725 

632 

756 

'5-54 

I8.5 

Closed 

1   Rodney  . 

1885 

90 

9.544 

474 

567 

16.83 

2O.  I 

Fire 

I    Mersey    . 

1885 

no 

6,628 

306 

399 

16.61 

21-7 

Rooms. 

[  Scout 

1885 

120 

3.370 

174 

207 

16.28 

19-3 

The  weight  of  boiler  includes  water,  funnel,  uptakes,  fittings,  spare  gear,  etc. 
Steam  blast  was  used  in  the  case  of  the  Colossus. 

Among  the  advantages  of  the  closed  fire-room  system  is  that  of  preventing 
all  escape  of  flame  and  smoke  to  the  fire  room,  the  leakage  being  all  inward  to 
the  fires.  The  objection  is  frequently  brought  against  this  system  that  its  use 
is  injurious  to  the  boilers,  tending  to  cause  leakage  at  the  tube  plate  when  the 
doors  are  opened.  This  is  generally  considered  to  be  due  to  the  chilling  action 
of  the  comparatively  cold  air,  which  thus  rushes  directly  through  the  furnace 
chamber  to  the  tubes.  The  leakage  which  sometimes  occurs  appears  to  be  due 
to  the  buckling  of  the  tube  plate,  whereby  the  joints  between  the  tubes  and  tube 
plate  are  opened  up,  as  a  result  of  the  unequal  contraction  under  the  chilling 
effect  of  the  air. 

The  attempt  has  been  to  palliate  rather  than  overcome  this  difficulty  by  the 
introduction  of  protecting  ferrules  at  the  tube  ends.  This  tendency  to  leakage 
under  heavy  air  pressure  should  not  of  necessity  be  charged  against  the  method 


220  MECHANICAL    DRAFT. 

of  producing  the  draft.  "  Because,"  as  stated  by  the  experienced  marine-boiler 
builder,  Mr.  A.  F.  Yarrow,1  "  a  certain  boiler  does  not  stand  a  given  air  pressure 
without  the  tubes  leaking  only  proves  that  this  air  pressure  is  too  much  for  that 
boiler,  but  does  not  prove  that  this  air  pressure  is  too  much  for  every  boiler, 
especially  in  face  of  the  fact  that  locomotive  engines  are  working  all  over 
the  world  at  air  pressures  varying  from  3  inches  to  8  inches."  There  can 
be  no  question  that  the  problem,  when  fairly  considered,  can  be  solved  so  as 
to  avoid  any  trouble  from  this  source.  It  can  to  a  great  degree  be  overcome 
by  an  arrangement  of  dampers  and  fire  doors,  such  that  the  draft  may  be  shut 
off  while  the  doors  are  open. 

Induced  System.  —  This  system,  whereby  a  partial  vacuum  is  produced  within 
the  furnace,  is  substantially  the  same  in  its  effect  as  a  chimney,  which  it  most 
closely  imitates  in  its  action.  It  possesses,  however,  not  only  greater  intensity, 
but  other  advantages  that  do  not  pertain  to  the  chimney.  In  fact,  it  is  the  most 
natural  method  of  applying  mechanical  draft,  there  is  no  change  in  arrange- 
ment of  the  boilers  from  that  which  obtains  when  a  chimney  is  used,  and  there 
can  be  no  question  as  to  its  adaptability  when  consideration  is  given  to  the  high 
pressure  differences  and  rates  of  combustion  which  are  secured  in  a  similar 
manner  in  locomotive  practice. 

It  is,  on  the  whole,  better  subject  to  control  than  the  other  systems  ;  its  leak- 
age is  always  inward,  avoiding  inconvenience  from  flame  and  smoke  at  the  fire 
doors,  which,  however,  is  only  liable  to  occur  under  heavy  air  pressures  and 
when  proper  damper  arrangements  are  lacking.  On  shipboard  it  produces 
excellent  ventilation  with  open  fire  rooms,  thereby  reducing  their  temperature. 
It  is  cleanly,  lends  itself  readily  to  control  by  the  dampers  which  may  be  intro- 
duced for  the  purpose,  and  can  by  simple  means  be  rendered  absolutely  auto- 
matic, requiring  no  attention  whatever  from  the  fireman. 

For  all  classes  of  work,  except  possibly  on  shipboard,  in  which  the  construc- 
tion may  prevent,  it  is  well  adapted,  it  usually  being  possible  to  place  the  fans 
above  the  boiler,  or  elsewhere  overhead,  in  such  a  manner  as  to  economize  floor 
space. 

The  early  objection  to  this  system,  before  it  had  been  extensively  adopted,  was 
that  the  fans  could  not  stand  the  high  temperature  of  the  gases  passing  through 
them.  The  best  refutation  of  this  statement  lies  in  the  fact  that  large  numbers 
of  the  Sturtevant  fans  have  been  running  for  years  under  these  conditions, 


iBoiler  Construction  Suitable  for  Withstanding  the  Strains  of  Forced  Draft  so  far  as  it 
Affects  the  Leakage  of  Boiler  Tubes.  Transactions  Institution  of  Naval  Architects.  London, 
1891. 


MECHANICAL    DRAFT. 


handling  gases  from  300°  to  500°  in  temperature,  and  even  higher.  Of  course 
these  fans  require  to  be  of  special  construction  to  withstand  the  heat,  and  must 
be  provided  with  means  for  keeping  the  bearings  cool.  But  these  features  were 
long  ago  introduced  by  this  Company,  and  today  the  decision  between  a  forced 
or  an  induced  system  is  to  be  made  independently  of  the  question  of  durability 
of  the  fans. 

The  inherent  relative  efficiencies  of  the  forced  and  induced  systems  are  diffi- 
cult of  determination  because  of  the  many  modifying  circumstances  which  exert 
their  influence.  Certain  comparative  tests  have,  however,  been  made,  princi- 
pally upon  marine  boilers  ;  but  they  can  hardly  be  taken  as  absolutely  conclu- 
sive. Among  them  are  those  conducted  at  the  Portsmouth  (England)  Dock 
Yard1  in  1890,  the  results  of  which  are  summarized  in  Table  No.  118.  Both 
tests  were  made  on  the  same  boiler,  the  induced  draft  being  dismantled  and 
replaced  by  the  forced  draft.  To  the  decreased  tendency  of  induced  draft  to 
blow  holes  through,  as  compared  with  forced  draft,  and  the  better  regulation 
and  distribution  of  the  air,  may  undoubtedly  be  attributed  a  part  of  its  supe- 
riority. It  is  also  claimed  that  the  action  of  the  air  and  gases  upon  entering 
the  tube  ends  is  such  that  with  forced  draft  a  contracted  vein  is  formed  which 
prevents  close  contact,  while  with  induced  draft  the  partial  vacuum  in  the  tubes 
causes  the  flow  to  be  uniform  across  the  section  of  the  tube,  and  thereby  insures 
better  contact  and  readier  abstraction  of  heat. 

Table  No.   118. —  Results  of  Experiments  at  Portsmouth  Dock  Yard  with  Boilers  of 
H.  M.  S.  Polyphemus. 


i 

i 

Temper- 
ature. 

1 

1 
I 

Lbs.  Water 
Evaporated 
per  Lb.  Coal. 

&S 

Lbs.  Water  Evap- 
orated per  Hour 
per  Sq.  Ft.  Grate. 

|ti 

ffi 

35 

^        (/i 

o       M 

>    -3 

^Jffi  MH 

i—  i 

Kind  of 
Draft. 

ration. 

1   1 
| 

•d 

I 

CU 

u    1? 

1* 

-,  2 

i 

ll 

* 

1 

^ 

|| 

H 

P 

1 

H 

H 

I* 

<§ 

H 

^ 

I 

Induced, 

96 

74.2 

620 

69.90 

80,600 

777>°44 

9.64 

11.13 

40.4 

389.6 

450.4 

426. 

Forced, 

96 

77-3 

51 

49.8 

94,500 

759.338 

8.03 

9-3 

47-3 

38l. 

444. 

395- 

The  induced  system  presents  an  opportunity  for  the  introduction  of  air  or 
water  heaters  which  are  to  abstract  the  heat  from  the  waste  gases,  thereby 
securing  one  of  the  greatest  possible  economies  in  the  modern  boiler  plant.  In 


Transactions  Institution  of  Naval  Architects.     London,  1895. 


222  MECHANICAL   DRAFT. 

fact,  no  plant  of  reasonable  size  should  be  equipped  without  a  heat  abstractor 
of  some  form  in  connection  with  the  fan.  And,  on  the  other  hand,  no  form  of 
abstractor  can  be  more  efficiently  installed  than  in  connection  with  a  fan  which 
possesses  the  ability  to  readily  overcome  the  increased  resistance. 

The  reduction  in  temperature  which  thus  results  not  only  increases  the  effi- 
ciency of  the  plant,  but  has  an  appreciable  effect  upon  the  proportions  of  the 
fan ;  for  upon  the  temperature  of  the  air  and  gases  which  pass  through  the  fan 
must  depend  its  size  and  speed  to  accomplish  the  desired  results  in  the  way  of 
draft  production.  In  a  fan  operating  under  the  plenum  or  forced  system,  the 
volume  of  air  supplied  to  the  fire  is  substantially  the  same  as  that  delivered  by 
the  fan,  making  no  allowance  for  leakage.  But  with  induced  draft  the  fan 
must  handle  a  volume  of  air  and  gases  which,  although  the  same  in  weight,  is 
greater  in  volume  practically  in  proportion  to  its  increase  in  absolute  tempera- 
ture. Disregarding  leakage,  the  weight  is  greater  than  the  air  admitted  by  the 
portion  of  the  coal  which  has  entered  into  chemical  combination  with  it.  On  a 
basis  of  1 8  pounds  of  air  per  pound  of  coal,  this  additional  weight  amounts  to 
5.5  per  cent,  while  with  a  supply  of  24  pounds  it  is  4.2  per  cent.  This  increased 
amount  may  enter  into  any  refined  calculations  of  fan  capacity,  but  it  is  unnec- 
essary to  go  into  the  refinement  of  making  allowance  for  difference  in  specific 
gravity  or  for  moisture  in  the  air  or  fuel.  0 

As  the  capacity  of  a  fan,  both  in  volume  handled  and  pressure  produced,  is 
easily  varied  by  a  change  in  speed,  sufficient  accuracy  is  secured  in  ordinary 
design  by  considering  that  air  is  the  fluid  handled,  and  that  the  general  rela- 
tions presented  in  Table  No.  113  will  exist  between  volume,  weight  and  pres- 
sure and  the  speed  and  power  of  a  fan  at  different  temperatures. 


CHAPTER  XI. 
ADVANTAGES  OF  MECHANICAL  DRAFT. 

General  Advantages.  — •  In  considering  the  advantages  of  mechanical  draft, 
comparison  is  obviously  to  be  made  with  the  chimney.  The  merits  of  the  me- 
chanical method  of  draft  production  may  be  discussed,  first,  broadly,  as  to  the 
general  economic  and  convenient  results,  and,  second,  in  detail,  as  to  the  par- 
ticular points  in  which  it  excels  chimney  draft.  In  all  cases  the  degree  of 
superiority  must,  of  course,  depend  upon  the  circumstances.  The  difference 
between  land  and  marine  requirements,  for  instance,  forms  an  important  factor. 
In  the  former,  the  practical  abolishment  of  the  chimney,  the  introduction  of  air 
or  water  heaters  and  the  more  perfect  combustion  stand  out  prominent  in  the 
advantages.  In  the  latter,  the  reduction  in  the  weight  of  the  boilers,  which  have 
always  been  among  the  heaviest  parts  of  the  machinery,  together  with  the 
combined  flexibility  and  economy  which  are  inherent  features  of  the  system,  are 
of  great  importance. 

Some  of  the  advantages  of  mechanical  draft  are  thus  summarized  by  Mr. 
Alfred  Blechyden : 1  "  First,  it  seems  fairly  well  established  that,  if  the  boilers 
are  well  constructed  and  are  provided  with  ample  room  to  ensure  circulation, 
their  steaming  power  may  without  injury  be  increased  to  about  30  to  40  per 
cent  over  that  obtained  on  natural  draft  for  continuous  working,  and  may  be 
about  doubled  for  short  runs.  Secondly,  such  augmentation  is  accompanied,  in 
normal  cases,  by  an  increased  consumption  per  indicated  horse-power.  But, 
thirdly,  the  same  or  even  greater  power  being  indicated,  it  may,  with  moderate 
assistance  of  forced  draft,  be  developed  with  a  smaller  expenditure  of  fuel,  the 
grates,  etc.,  being  properly  proportioned.  Fourthly,  forced  draft  enables  an 
inferior  fuel  to  be  used;  and,  fifthly,  under  certain  conditions  of  weather,  when 
with  normal  proportions  of  boiler  it  would  be  impossible  to  maintain  steam  with 
natural  draft,  the  normal  power  may  with  forced  draft  be  ensured.  In  particular 
cases  any  or  all  of  these  advantages  may  be  a  source  of  economy ;  and  the  first 
of  them  may  render  possible  that  which  would  otherwise  be  impracticable." 


i  A  Review  of  Marine  Engineering  during  the  Past  Decade.    Alfred  Blechyden.    Proceedings 
Institution  of  Mechanical  Engineers.     London,  1891. 


224  MECHANICAL    DRAFT. 

"Artificial  draft,"  so  states  Mr.  W.  S.  Hutton,1  "can  be  readily  adjusted  to 
effect  the  combustion  of  different  kinds  of  fuel  at  different  rates  of  combustion. 
It  permits  efficient  combustion  of  fuel  of  inferior  quality,  and  enables  a  steady 
supply  of  steam  to  be  maintained,  independent  of  climate  and  weather.  It  en- 
ables the  supply  of  air  to  be  properly  distributed  to  the  fuel  in  the  furnace  to 
effect  economical  combustion. 

"  The  supply  of  air  above  the  fuel  can  be  readily  adjusted  to  effect  combus- 
tion of  the  gases  evolved  by  the  fuel,  and  the  supply  of  air  below  the  fuel  can 
be  regulated  to  effect  the  combustion  of  the  solid  portion  of  the  fuel,  and  the 
movement  of  the  hot  gases  can  be  readily  controlled." 

There  is  no  valuable  feature  of  the  chimney  that  is  not  possessed  by  the  fan 
to  at  least  the  same,  and  in  many  cases  to  a  more  marked,  degree.  The  very 
features  which,  as  shown  in  the  preceding  pages,  are  most  conducive  to  economy 
are  those  which  are  incidental  to  the  use  of  a  fan  for  draft  production.  While 
the  recent  extensive  introduction  of  induced  draft  in  stationary  practice  has 
done  much  to  emphasize  the  advantages  of  this  system,  the  general  superority 
of  mechanical  draft,  properly  applied,  has  long  been  recognized  by  those  who 
have  given  careful  consideration  to  the  matter ;  but  never  has  the  entire  matter 
been  treated  in  its  collective  aspect,  as  it  is  here  presented. 

In  any  consideration  of  the  substitution  of  other  means  of  draft  production 
for  chimney,  the  steam  or  compressed  air  jet  may  be  primarily  admitted, 
although  it  has  been  shown  that,  from  the  standpoint  of  economy  of  operation, 
the  fan  must  be  substituted  for  them.  Nearly  forty  years  ago  Mr.  D.  K.  Clark2 
"  testified  to  the  advantage  of  a  rapid,  or  rather  intense  draft,  in  perfecting  com- 
bustion and  extinguishing  smoke,"  upon  which  Mr.  C.  Wye  Williamss  was  led  to 
remark:  "But  the  difficulty  lies  in  the  obtaining  of  this  'intense  draft.'  .  .  . 
The  absolute  command  of  draft  for  the  generation  of  the  required  quantity  of 
steam,  to  enable  the  -engines  to  work  to  their  full  power,  being  then  so  essential, 
it  becomes  a  question  whether  other  means  than  the  natural  draft  should  not  be 
resorted  to ;  since,  independently  of  the  uncertainty  in  the  amount  of  draft,  and 
the  consequent  irregularity  in  the  working  effect  of  the  engines,  the  cost  of 
sustaining  that  draft  may  be  so  much  in  excess  of  what  an  artificial  draft  would 
be."  M.  Peclet  also  at  this  time  investigated  the  subject,  showed  the  low  effi- 
ciency of  the  chimney  as  compared  with  a  fan,  and  recommended  the  use  of 
rotary  fans,  applied  for  exhausting  on  the  induced  system. 


*  Steam-Boiler  Construction.     Walter  S.  Hutton.    London,  1891. 

2  Combustion  of  Coal  and  the  Prevention  of  Smoke.     C.  Wye  Williams.     London, 

3  Ibid. 


MECHANICAL    DRAFT. 


225 


Rankine1  bases  his  calculations  of  results  with  forced  draft  on  an  air  supply 
of  only  18  pounds  of  air  per  pound  of  coal,  while  those  upon  chimney  draft  are 
based  upon  24  pounds,  and  then  remarks  that  "with  a  forced  draft  there  is  less 
air  required  for  dilution,  consequently  a  higher  temperature  of  the  fire,  conse- 
quently a  more  rapid  conduction  of  heat  through  the  heating  surface,  conse- 
quently a  better  economy  of  heat  than  there  is  with  a  chimney  draft."  So  Mr. 
D.  K.  Clark2  states  that  "  the  system  of  forced  draft  opens  the  way  for  increase 
of  efficiency  in  facilitating  the  adoption  of  grates  of  diminished  area  in  combi- 
nation with  acceleration  of  combustion." 

Messrs.  Mills  and  Rowan,3  writing  at  considerable  length  in  1889,  discuss 
the  subject  in  part  as  follows:  "The  principles  of  what  is  now  becoming 
well  known  under  the  name  of  '  forced  combustion  '  have  been  repeatedly  advo- 
cated during  past  years  by  those  who  have  devoted  thought  and  study  to  the 
subject.  The  position  assumed  by  them  —  which  is  now  finding  favor  amongst 
engineers  —  has  been,  in  brief,  that  the  air  supply  required  for  combustion  in 
furnaces  can  be  more  economically  furnished  by  mechanical  power  than  by  the 
action  of  chimneys ;  and  that  the  mechanical  method  has  other  advantages, 
which  enable  it  to  be  preferred  to  the  one  that  is  older,  but  more  imperfect. 
One  of  these  advantages  is  the  higher  temperature  of  combustion,  which  is 
equivalent,  with  a  boiler  of  good  design,  to  an  increased  evaporative  power  of 
boiler,  or  to  increased  evaporative  effect  for  the  fuel.  Another  advantage,  which 
has  not  been  fully  realized  in  any  plan  as  yet  introduced  in  practical  work,  is 
that  the  rate  of  travel  and  escape  of  the  flame  and  hot  products  of  combus- 
tion is  under  control.  It  is  thus  possible  to  cool  them  more  completely  than 
can  be  done  when  chimney  draft  is  used,  and  this  means  a  saving  of  heat  which 
would  otherwise  be  uselessly  dissipated. 

"  Mechanical  or  artificial  draft  thus  presents  to  us  a  method  of  economically 
furnishing  the  air  supply  to  furnaces  and  producing  a  more  efficient  combustion 
temperature,  while  it  also  renders  possible  further  economies  due  to  retarding 
the  movement  and  escape  of  hot  gases  and  to  preliminary  heating  of  the  air 
supply  by  waste  heat  or  otherwise." 

Mr.  W.  S.  Hutton-*  states  that  "the  economy  that  may  be  obtained  by  com- 
bustion with  forced  draft  in  a  steam  boiler  is  due  to  the  increased  rate  of  com- 
bustion and  the  increased  efficiency  of  the  heating  surfaces  produced  by  it, 


1  The  Steam  Engine  and  Other  Prime  Movers.     W.  J.  M.  Rankine.     London,  1885. 

2  The  Steam  Engine.     D.K.Clark.     London,  1890. 

3  Chemical  Technology.     E.  J.  Mills  and  F.  J.  Rowan.     Vol.  I.     London,  1889. 

4  Steam-Boiler  Construction.     Walter  S.  Hutton.     London,  1891. 


226 


MECHANICAL   DRAFT. 


resulting  in  increased  boiler  power.  The  increase  of  power  obtained  depends 
principally  upon  the  quantity  of  air  brought  into  intimate  contact  with  the  fuel 
in  a  given  time,  but  the  power  of  the  boiler  may  generally  be  increased  from  40 
to  100  per  cent  by  the  application  of  well-arranged  forced  draft." 

Viewed  from  the  standpoint  of  the  economic  results  to  be  obtained  by  the 
introduction  of  air  heaters  —  in  this  case  the  Marland  apparatus — and  the  sub- 
stitution of  a  blower  for  a  chimney,  Mr.  J.  C.  Hoadley1  states  that  "there  can 
be  no  doubt  that  the  heat  to  be  returned  to  the  furnace  would  several  times  ex- 
ceed that  necessary  to  make  the  power  required  to  drive  the  exhausting  fan,  to 
the  operation  of  which  the  final  temperature  of  the  gases  presents  no  objection. 
...  In  the  construction  of  new  works  the  outlay  for  the  Marland  apparatus 
will  be,  or  at  least  may  be,  largely  offset  by  the  saving  in  the  cost  of  a  chimney." 

In  its  relation  to  marine  service,  Mr.  J.  W.  C.  Haldane2  points  out  that  "  the 
employment  of  forced  draft  does  not  involve  any  new  principles  in  chemistry, 
but  enables  us  with  greater  certainty  to  carry  out  those  conditions  which 
chemistry  shows  are  essential  to  produce  perfect  combustion,  and  thus  obtain 
higher  efficiency  per  pound  of  fuel  than  can  be  obtained  by  natural  draft."  As 
indicating  conclusively  the  economy  of  forced  draft  in  marine  practice,  Mr. 
Haldane  presents  data,  here  given  in  Table  No.  119,  from  the  more  elaborate 
classification  of  Mr.  Blechyden.  From  a  commercial  standpoint  the  decreased 
heating  surface  per  horse-power  is  only  second  in  importance  to  the  increased 
effciency  per  pound  of  coal. 

The  Engineering  Record,3  editorially  commenting  upon  the  matter  of  mechani- 
cal draft,  asserts  that  "there  are  important  advantages  possessed  by  mechanical 
draft  which  are  sure  to  make  a  place  for  it  and,  we  believe,  bring  about  its 


Table  No.   119. — Comparison  of  Results  under  Natural  and  Forced  Draft. 


*t 

Heating  Surface. 

Coal  Burned 

AVERAGES. 

It 
!« 

Perl.  H.  P., 
in  Square 

Per  Pound 
of  Coal  per 
Hour  in 

I.  H.  P. 

per  Square 
Foot  of  Grate. 

Foot  of  Grate 
per  Hour, 
in  Pounds. 

per  I.  H.  P. 
per  Hour, 
in  Pounds. 

"o 

Feet. 

Square  Feet. 

For  all  steamers 

28 

3-275 

2.14 

11.22 

17.08 

1.522 

For  natural  draft 

22 

3-56° 

2.25 

8.91 

13.92 

1-573 

For  forced  draft 

6 

2.412 

1.72 

20.98 

28.15 

1-336 

1  Warm-Blast  Steam-Boiler  Furnace.     J.  C.  Hoadley.     New  York,  1886. 

2  Steamships  and  Their  Machinery.     J.  W.  C.  Haldane.     London  and  New  York,  1893. 

3  The  Engineering  Record.     New  York,  Jan.  6,  1894. 


MECHANICAL   DRAFT.  227 

extended  application.  The  flexibility  of  a  draft  controlled  by  power  is  a  most 
valuable  feature.  By  the  momentary  increase  in  the  motive  power  applied  to 
the  fan  almost  any  desired  increase  of  capacity  can  be  obtained,  and  sudden  or 
unusual  calls  can  be  met  by  simply  an  extra  turn  of  the  throttle  valve  which 
controls  the  fan  engine.  Mechanical  draft  enables  feed-water  heaters  or  econo- 
mizers to  be  used  in  the  flue  and  the  waste  of  escaping  gases  utilized.  Indeed, 
these  two  appliances  go  hand  in  hand,  either  one  requiring  the  other  for  the 
best  practical  efficiency.  Mechanical  draft  has  no  objectionable  features  on 
account  of  expense  of  the  plant  in  first  cost.  A  boiler  plant,  fitted  with  econo- 
mizer, fan  and  short  smokestack,  can  be  installed,  it  is  said,  with  a  smaller 
expenditure  of  money  than  a  plant  of  equal  capacity  without  an  economizer 
and  provided  with  a  good  brick  chimney,  say  125  feet  high.  The  saving  in 
fuel  due  to  the  operation  of  the  economizer  is  thus  nearly  all  net  gain.  The 
application  of  power  for  the  production  of  draft  is  an  innovation  in  steam- 
engineering  practice  which  seems  bound  to  become  generally  endorsed  in  the 
near  future." 

Much  more  might  be  quoted  in  the  same  tenor  from  other  sources,  but  that 
which  has  been  here  presented  certainly  appears  sufficient  to  prove  that 
mechanical  draft  has  the  unqualified  endorsement  of  those  who  have  investi- 
gated its  merits.  Although  its  efficiency  has  long  been  recognized,  its  recent 
rapid  progress  is  the  best  evidence  that,  along  with  higher  steam  pressures, 
more  rapidly  running  engines  and  increased  engine  and  boiler  efficiency, 
mechanical  draft  must  take  its  stand  as  a  most  important  factor  in  the  accom- 
plishment of  economic  results. 

The  remainder  of  this  chapter  will  b$  devoted  to  a  summary  of  the  specific 
advantages  of  mechanical  draft.  In  the  preceding  chapters  it  has  been  the 
endeavor  to  show  the  principles  upon  which  its  success  and  superiority  depend, 
to  point  out  wherein  its  advantages  lie,  and  to  present  conclusive  evidence  to 
substantiate  all  statements.  Additional  evidence  is  introduced  in  certain  cases 
which  follow,  and  the  closing  chapter  presents  numerous  examples  of  the  man- 
ner of  application  of  the  Sturtevant  fans,  together  with  the  results  which  have 
been  obtained  in  practice. 

Evidently,  this  is  a  book  with  a  purpose,  and  that  purpose  is,  first  and  all-im- 
portant, to  convince  the  reader,  by  facts  and  substantiated  claims,  that  under  all 
ordinary  conditions  mechanical  draft  is  preferable  to  chimney  draft ;  and,  second, 
that  this  Company  is,  through  its  extended  experience  and  large  manufacturing 
facilities,  in  a  position  to  introduce  the  best  apparatus  in  the  most  intelligent 
manner.  To  a  careful  consideration  of  the  following  claims  the  reader  is  there- 
fore invited. 


228  MECHANICAL   DRAFT. 

Necessity.  —  In  its  broadest  sense,  the  necessity  of  mechanical  draft  is  to  be 
measured  upon  a  commercial  basis.  If  by  its  introduction  greater  economy  in 
the  first  cost  or  running  expense  of  a  steam  plant  may  be  secured,  or  even  if  in 
its  operation  it  is  more  convenient  and  adaptable  to  the  conditions,  it  may  rea- 
sonably be  considered  as  a  commercial  necessity.  But  there  are  conditions 
under  which  the  introduction  of  a  chimney  would  be  impractical,  or  it  would  be 
extremely  difficult  to  secure  the  desired  results  without  the  aid  of  mechanical 
draft.  Thus,  for  instance,  the  draft  required  might  necessitate  a  chimney  of 
excessive  height  and  cost ;  it  might  be  impossible  to  further  increase  the  capac- 
ity of  an  old,  or  erect  a  new,  chimney ;  or  the  stability  of  the  ground  might  be 
such  as  to  make  it  undesirable  to  erect  a  chimney  thereon.  Under  these  condi- 
tions mechanical  draft  becomes  a  veritable  necessity. 

On  shipboard,  "  it  is  perhaps  not  too  much  to  say  that  the  very  high  speeds 
that  have  been  recently  obtained  by  several  cruisers  of  moderate  dimensions 
would  have  been  impracticable  without  the  application  of  forced  draft."  J  In 
fact,  forced  draft  in  ships  of  war  is  now  recognized  as  a  necessity.  This  is  par- 
ticularly true  in  the  case  of  small  vessels  of  the  torpedo-boat  type,  in  which  the 
excessively  high  rate  of  combustion  and  the  intensity  of  draft  are  such  as  would 
demand  a  chimney  of  such  excessive  height  as  to  be  absolutely  impracticable  in 
a  vessel  of  this  description. 

Adaptability,  —  The  chimney  requires  certain  fixed  and  practically  unaltera- 
ble conditions  for  its  location  and  erection,  and,  once  erected,  is  only  to  a  lim- 
ited extent  adaptable  to  changes  in  the  requirements.  The  fan,  as  employed 
for  mechanical  draft,  may,  however,  be  adapted  to  a  great  variety  of  conditions  ; 
for  the  character  of  its  construction,  usually  of  steel  plate,  makes  it  possible  to 
build  it  to  meet  almost  any  conceivable  requirements.  If  restricted  space  abso- 
lutely demands,  it  may  be  made  of  comparatively  small  size  and  provided  with 
an  engine  of  extra  power  to  run  it  at  high  speed.  It  can  be  placed  above  the 
boilers  or  suspended  overhead,  thus  occupying  no  valuable  floor  space,  as  is  the 
case  with  the  chimney  ;  or  it  may  be  placed  in  any  convenient  location  upon 
the  floor,  or  even  in  an  adjoining  apartment,  and  connection  made  to  or  from 
the  boilers  by  means  of  an  underground  or  overhead  duct.  At  the  same  time 
the  size  or  shape  may  be  made  such  as  to  best  adapt  it  to  the  location.  Under 
no  circumstances  are  the  foundations  necessary  for  the  proper  installation  of  a 
mechanical-draft  plant  to  be  compared  in  expense  with  those  which  are  required 
for  a  chimney. 


i  Closed    Stokeholds.      Richard    Sennett.      Transactions    Institution   of    Naval    Architects. 
London,  1886. 


MECHANICAL   DRAFT.  229 

If  the  conditions  are  changed  the  same  fan  may  frequently  be  made  to  meet 
the  requirements  byva  change  of  location,  arrangement  or  motive  power.  Thus, 
while  a  chimney  cannot  without  great  expense  be  increased  in  its  capacity,  the 
mere  lengthening  of  the  cut-off  of  the  engine,  or  possibly  its  exchange  for  one 
of  larger  size,  is  all  that  may  be  necessary  to  adapt  a  fan  to  the  new  conditions. 
This  is  particularly  true  of  an  increase  in  the  boiler  plant. 

The  peculiar  adaptability  of  the  fan  to  the  assistance  of  a  chimney  is  of  great 
importance,  for  by  its  use  many  a  plant  weak  in  chimney  power  has  been 
brought  up  to  the  requirements.  It  has  likewise  proved  its  great  value  in  mak- 
ing possible  the  use  of  economizers  or  other  heat  abstractors  both  in  old  and 
new  plants.  Its  instantaneous  adaptability  to  a  change  in  the  demand  for  steam 
renders  it  of  particular  value  wherever  this  is  a  characteristic  of  the  plant. 

While  with  the  chimney  the  draft  must  always  be  produced  by  suction,  in  the 
case  of  the  fan  both  forced  and  induced  draft  are  available,  according  to  the 
requirements  of  the  case.  The  fan  may  be  driven  at  exactly  the  speed  adapted 
to  the  most  economical  operation  of  the  boiler  plant,  and  may  be  instantly  and, 
if  desired,  automatically  adapted  to  other  conditions. 

A  fan  may  be  temporarily  employed  for  the  production  of  draft  in  the  case  of 
a  plant  whose  location  is  not  fixed,  or  during  alterations  or  removal.  The  erec- 
tion of  a  chimney  for  temporary  use  is  hardly  to  be  thought  of. 

Controllability.  —  One  of  the  most  important  of  the  advantages  possessed  by 
mechanical  draft  is  the  perfect  control  which  may  be  maintained  over  its  action. 
Such  control,  in  the  case  of  a  chimney,  rests  principally  in  the  operation  of 
dampers  which  restrict  the  air  flow,  but  with  a  given  temperature  do  not  affect 
the  intensity  or  pressure  of  the  draft.  Proper  control  of  a  fan  consists  in  so 
regulating  its  speed  —  usually  with  dampers  constantly  open  —  that  not  only  are 
changes  produced  in  the  air  volume  moved,  but  the  pressure  is  varied  in  such 
degree  as  to  overcome  the  resistances.  Where  the  rate  of  combustion  is  prac- 
tically constant,  such  control  can  be  secured  to  a  reasonable  extent  by  manipu- 
lation of  the  throttle  valve.  But  for  more  variable  conditions  and  for  all  close 
regulation  the  throttle  valve  on  the  engine  is  operated  by  an  Automatic  Draft 
Regulator,  a  device  so  constructed  that  the  throttle  is  opened  as  the  steam 
pressure  falls,  and  vice  versa.  The  result  is  almost  absolute  uniformity  of  pres- 
sure, as  is  shown  by  steam-pressure  records  presented  upon  succeeding  pages. 

With  a  chimney  the  intensity  of  the  draft  depends  upon  the  intensity  of  the 
fire,  and  is,  therefore,  least  when  the  fire  is  low,  which  is  usually  the  time  when 
it  is  most  required.  With  the  fan,  on  the  other  hand,  it  is  possible  to  instantly 
produce  practically  the  maximum  draft  under  these  conditions.  The  control 
which  is  thus  maintained  over  mechanical  draft  is  of  special  value  when  it  is 


23o  MECHANICAL    DRAFT. 

desired  to  start  up  quickly  fires  which  have  been  banked.  This  feature  is  par- 
ticularly applicable  on  shipboard,  where  steam  may  be  brought  almost  instantly 
up- to  the  blowing-off  point  from  banked  fires  when  the  vessel  casts  off  its  moor- 
ings, thereby  obviating  the  noise  of  blowing  off  previous  to  starting.  In  the 
naval  marine  the  importance  is  more  marked,  for  the  vessels  may,  in  the  case 
of  an  anticipated  engagement,  be  kept  ready  to  respond  at  an  instant's  notice ; 
all  that  is  necessary  to  this  response  being  the  opening  of  the  steam  valves 
upon  the  fan  engines. 

Flexibility.  —  This  advantage  possessed  by  mechanical  draft  is  closely 
related  to  controllability ;  yet  it  is  an  essential  feature  of  this  device,  which  is 
only  to  a  limited  extent  possessed  by  the  chimney.  The  chimney  is,  in  its  very 
stability,  the  opposite  of  flexible.  True  it  is  that  the  amount  of  air  handled  by 
a  chimney  can  be  restricted  by  dampers,  but  the  intensity  of  its  draft  is  fixed  by 
its  height  and  cannot  be  changed  to  suit  the  conditions.  The  fan,  on  the  other 
hand,  being  under  constant  control,  is  thoroughly  flexible  as  to  both  volume  and 
intensity  of  draft.  Accordingly,  in  an  already  established  plant,  the  fan  is 
always  ready  to  respond  to  a  change  in  the  requirements.  In  electric  railway 
and  lighting  service  this  feature  is  of  the  greatest  value,  as  it  makes  possible 
instant  response  to  sudden  demands.  Likewise,  in  marine  service,  particularly 
in  the  navy,  flexibility  in  the  draft  apparatus  keeps  it  constantly  suited  to  the 
conditions,  and  always  in  a  position  to  readjust  itself.  As  stated  by  Chief 
Engineer  William  H.  Shock, '  "  it  can  be  readily  adjusted  for  the  combustion  of 
different  kinds  of  fuel  and  for  widely  different  rates  of  combustion,  so  that  a 
given  boiler  may  be  worked  under  greatly  varying  conditions."  As  already 
shown,  the  power  of  a  boiler  may  "be  increased  from  40  to  100  per  cent  by 
the  application  of  well-arranged  forced  draft."  On  the  other  hand,  this  feature 
of  flexibility  makes  it  possible  in  most  cases  to  largely  increase  the  size  of  a 
steam  plant  before  exceeding  the  possible  capacity  of  the  mechanical  draft 
apparatus. 

The  automatic  action  of  the  fan,  whereby  increased  resistance  tends  to  in- 
crease the  speed  to  overcome  that  resistance,  is  a  prominent  feature  of  its  flex- 
ibility, which  is  absolutely  lacking  in  the  chimney. 

Independence  of  Climatic  Conditions.  —  The  influence  of  a  change  in  the 
temperature  of  the  external  atmosphere  upon  the  draft  of  a  chimney  has  already 
been  shown  in  Table  No.  107.  This  influence  is  so  great  as  to  make  a  decided 
difference  between  the  draft  in  summer  and  that  in  winter.  On  shipboard,  in 
particular,  the  intensity  of  the  wind  also  affects  the  draft  to  such  an  extent  as 


Steam  Boilers.     William  H.  Shock.     New  York,  1880. 


MECHANICAL   DRAFT.  231 

to  frequently  cause  considerable  inconvenience.  The  influence  of  damp  and 
muggy  days  is  everywhere  recognized  in  its  effect  of  deadening  the  fires.  All 
these  results  are  likely  to  occur  when  the  chimney  is  the  sole  reliance  for  draft 
production. 

But  the  fan  always  operates  independently  of  climatic  conditions ;  the  draft 
can  be  made  as  strong  in  summer  as  in  winter,  and  on  a  muggy  day  as  on  one 
that  is  bright  and  clear.  This  latter  feature  is  of  almost  inestimable  value  in 
electric  railway  plants ;  for  it  is  on  just  such  days,  perhaps  made  worse  by  snow 
and  slush,  that  the  greatest  demand  is  made  for  power,  while  the  electricity  is 
most  rapidly  dissipated,  and  with  a  chimney  the  increased  demand  for  steam  is 
least  readily  met.  The  positive  character  of  the  fan,  which  is  thus  displayed, 
renders  it  available  at  all  times  and  independent  of  all  external  influences. 
This  feature  is  very  clearly  evidenced  by  practical  experience,  as  expressed  by 
Mr.  Alfred  Blechyden:1  "Under  certain  conditions  of  weather,  when  with  nor- 
mal proportions  of  boiler  it  would  be  impossible  to  maintain  steam  for  the 
ordinary  speed  with  natural  draft,  the  normal  power  may  with  forced  draft  be 
ensured." 

Portability.  —  A  brick  chimney,  once  erected,  is  a  fixture ;  and  even  a  steel- 
plate  stack  is  practically  non-portable,  considering  the  difficulty  and  expense  of 
its  removal.  Accordingly,  the  only  resort,  in  case  of  change  of  location,  is  to 
leave  the  chimney  as  the  buildings  are  left.  This  holds  true  even  if  the  boilers 
are  only  to  be  removed  to  another  portion  of  the  same  works.  The  chimney 
must  remain  ;  it  is  only  suited  to  certain  conditions,  and  is  practically  valueless 
unless  those  conditions  exist. 

On  the  other  hand,  the  fan  and  its  accompanying  engine,  arranged  for  the 
mechanical  production  of  draft,  are  always  readily  portable.  They  may  be 
removed  to  another  part  of  the  works,  or  to  a  distant  place,  with  almost  equal 
facility,  and  not  infrequently  may  be  set  up  again  under  decidedly  different  con- 
ditions; while  the  chimney  would  have  to  be  abandoned  altogether  under  similar 
circumstances.  The  advantage  of  mechanical  draft  under  such  circumstances 
is  well  evidenced  in  the  experience  of  this  Company,  the  original  chimney,  built 
years  ago,  being  left  as  a  useless  incumbrance  when,  through  force  of  circum- 
stances, the  boilers  were  removed  to  another  location.  Mechanical  draft  was 
here  applied  and  the  gases  discharged  through  a  short  stack,  which  in  its  rela- 
tion to  the  useless  chimney  is  graphically  presented  in  the  reproduction  of  a 
photograph  in  Chapter  XIII. 


i  A  Review  of  Marine  Engineering  during  the  Past  Decade.     Alfred  Blechyden.     Proceed- 
ings Institution  of  Mechanical  Engineers.     London,  1891. 


23 2  MECHANICAL   DRAFT. 

Salability.  —  Closely  linked  to  and  dependent  upon  the  feature  of  portabil- 
ity is  the  facility  with  which  a  fan,  previously  employed  for  draft  production, 
may  be  sold,  not  of  course  at  its  original  price,  but,  particularly  in  the  case  of 
small  fans  complete  in  themselves,  at  a  price  that  is  worthy  of  consideration 
alongside  of  the  continuing  fixed  charges  upon  a  chimney  that  has  been  aban- 
doned. For  experimental  or  temporary  purposes  a  fan  may  even  be  hired  at  a 
reasonable  rate,  and  its  employment  may  avoid  an  absolute  expenditure  for 
stack  or  chimney. 

The  feature  of  salability  is  of  especial  importance  when  a  plant  is  to  be 
altered  —  increased,  for  instance  —  and  a  new  fan  of  different  capacity  is  re- 
quired. The  fan  is  always  an  available  asset.  Certainly  such  a  feature  should 
enter  into  any  consideration  of  the  best  method  of  draft  production. 

Efficiency  of  Fan  vs.  Chimney.  —  The  fact  that  the  chimney  is  an  exceedingly 
inefficient  device  for  moving  air  has  already  been  made  clear  in  Chapter  IX., 
and  it  has  also  been  shown  that  a  fan  requires  so  little  power  by  comparison 
that,  considered  simply  as  the  means  by  which  air  is  to  be  moved,  the  fan  pos- 
sesses advantages  far  and  above  the  chimney.  As  also  there  shown,  the  adop- 
tion of  the  fan  renders  available  the  greater  part  of  the  heat  necessary  to  the 
operation  of  the  chimney,  and  presents  the  greatest  opportunity  for  increased 
efficiency  which  exists  in  modern  boiler  practice.  In  fact,  in  this  one  feature  of 
efficiency  of  the  fan  is  to  be  found  the  source  of  its  great  economic  superiority. 
A  still  further  increase  in  the  efficiency  of  mechanical  draft  results  from  the 
utilization,  for  feed-water  or  other  heating,  of  the  exhaust  steam  from  the  fan 
engine,  thereby  practically  extinguishing  the  item  of  cost  of  operation. 

As  previously  stated,  the  amount  of  steam  that  must  be  supplied  for  the  ope- 
ration of  the  fan  depends  upon  the  character  and  size  of  the  plant.  Evidently, 
the  greatest  proportional  supply  will  be  required  when  small  fans  are  run  at 
high  speed  and  under  excessive  air  pressure.  These  are  the  conditions  encoun- 
tered in  the  forced-draft  contract  trials  of  naval  vessels.  Nevertheless,  in  the 
case  of  12  vessels  of  the  United  States  Navy,1  of  various  types  and  sizes,  most 
of  them  equipped  with  Sturtevant  fans,  the  average  indicated  horse-power  of 
the  blower  engines  averaged  only  1.2  per  cent  of  the  total  indicated  horse-power 
of  the  main  engines  under  the  conditions  of  full-power  forced-draft  contract 
trials.  The  corresponding  average  air  pressure  in  the  boiler  rooms  or  ashpits 
was  2.1  inches,  and  the  average  rate  of  combustion,  so  far  as  given,  was  about 
40  pounds  of  coal  per  square  foot  of  grate  per  hour. 


i  Machinery  of  the  New  Vessels  of  the  United  States  Navy.     George  W.  Melville.     Trans- 
actions Society  of  Naval  Architects  and  Marine  Engineers.     1893. 


MECHANICAL    DRAFT.  233 

Omission  of  Chimney.  —  While  the  absence  of  the  chimney  is  the  natural  con- 
sequence of  the  introduction  of  mechanical  draft,  and  therefore  a  source  of 
economy,  there  are  conditions  under  which  its  omission  may  be  a  direct  advan- 
tage in  itself,  regardless  of  economic  considerations.  This  is  particularly  true 
in  the  case  of  torpedo  and  similar  small  boats,  as  already  indicated,  where  a 
stack  of  proper  dimensions  to  produce  the  draft  would  be  out  of  the  question. 

Increased  Rate  of  Combustion.  —  Independently  of  the  greater  economy  with 
high  rates  of  combustion,  mechanical  draft  stands  as  the  only  means  by  which 
the  increased  rate  may  be  economically  obtained.  Coincidently  the  boiler  ca- 
pacity must  of  necessity  be  greater,  provided  the  grate  area  is  maintained.  The 
expense  or  inconvenience  of  a  chimney,  to  obtain  rates  above  20  or  25  pounds 
per  square  foot  per  hour,  becomes  so  great  as  to  practically  preclude  an  increase. 
As  observed  by  Mr.  A.  J.  Durston,1  "as  long  as  draft  was  dependent  on  a  funnel 
for  its  production  a  much  greater  combustion  than  25  pounds  of  coal  per  square 
foot  of  grate  was  rarely  achieved ;  with  artificial  draft,  on  the  other  hand,  the 
rate  of  combustion  may  be  accelerated  to  any  amount,  and  as  a  boiler's  capability 
of  transmitting  heat  without  injury  to  itself  is  simply  a  matter  of  degree,  ex- 
perience has  been  necessary  to  determine  the  rates  of  combustion  that  can  with 
safety  be  employed  with  different  types  of  boilers."  When  it  is  considered  that 
in  boilers  of  the  marine  type  the  combustion  rate  resulting  from  the  employment 
of  mechanical  draft  is  now  carried  as  high  as  40  to  50  pounds,  that  in  torpedo- 
boat  and  similar  service  a  rate  of  70  to  80  pounds  is  frequent,  and  in  locomotive 
practice  as  high  as  120  pounds  is  not  at  all  unusual,  the  possibilities  of  increased 
rates  of  combustion  with  mechanical  draft  are  evident. 

When  the  capacity  of  a  boiler  can  be  increased  from  40  to  100  per  cent  by 
the  application  of  mechanical  draft,  with  the  consequent  higher  combustion  rate 
as  already  shown,  there  can  be  no  question  as  to  its  desirability. 

Efficiency  of  Combustion. —  It  has  already  been  clearly  demonstrated  in 
Chapter  VII.  that  combustion  may  be  more  economically  accomplished  with  high 
than  with  low  rates  of  combustion.  For  this  reason  it  is  common  practice,  in 
the  introduction  of  mechanical  draft,  to  so  reduce  the  grate  area  as  to  decidedly 
increase  the  rate  per  square  foot  necessary  to  maintain  the  total  rate  previously 
existing  with  the  larger  grate.  The  thicker  fires  required,  the  better  utilization 
of  the 'air  supplied,  the  higher  temperature  and  the  ability  of  mechanical  draft 
to  create  the  pressure  difference  necessary  to  these  results  have  already  been  in- 
dicated as  the  elements  of  economy  in  higher  combustion  rates. 


i  Some  Notes  on  the  History,  Progress  and  Recent  Practice  in  Marine  Engineering.     A.  J. 
Durston.     Transactions  Institution  of  Naval  Architects.     London,  1892. 


234  MECHANICAL    DRAFT. 

This  fact  is  recognized  by  Chief  Engineer  William  H.  Shock1  in  his  state- 
ment that  "  artificial  draft  has  the  great  advantage  that,  all  things  considered,  it 
is  cheaper  than  natural  draft  for  high  rates  of  combustion."  Hutton,2  basing 
his  statement  upon  the  experiments  of  Spence,  also  asserts  that  "by  the  use  of 
moderate  forced  draft  a  higher  efficiency  of  combustion  is  obtainable  than  by 
using  natural  draft  only."  But  efficiency  is  not  to  be  measured  alone  by  the 
economic  combustion  of  the  fuel ;  it  must  also  include  the  commercial  efficiency 
of  the  entire  plant.  If,  therefore,  as  is  evident,  the  capacity  of  a  given  plant 
may  be  greatly  increased  by  the  introduction  of  mechanical  draft  and  its  acces- 
sories, the  fixed  charges  for  a  given  evaporation  will  be  decreased  and  the 
aggregate  efficiency  will  be  raised.  The  importance  of  an  increased  combustion 
rate  in  the  accomplishment  of  such  efficiency  is  well  presented  by  Mr.  F.  Gross,s 
who  makes  the  statement  that  "the  special  advantage  is,  to  give  the  same  econ- 
omy when  burning  at  twice  the  rate  in  half  the  number  of  boilers,  or  to  make 
the  steam  as  economically  as  is  now  done  with  natural  draft  and  plain  tubes 
when  burning  three  times  the  rate  in  one-third  the  number  of  natural-draft 
boilers." 

Burning  Cheap  Fuels.  — The  ability  to  utilize  cheap  fuels,  which  is  an  inherent 
advantage  of  mechanical  draft,  has  already  been  pointed  out  in  Chapter  V.  It 
was  there  shown  that  in  a  certain  plant  of  1,005  horse-power  the  introduction  of 
the  Sturtevant  mechanical  draft  plant  had  resulted  in  a  weekly  saving  of  $126.00 
in  the  fuel  account.  Further  instances  of  economy  resulting  from  the  production 
of  draft  by  mechanical  means  are  presented  in  Chapter  XIII.  The  intensity  of 
draft  required  in  the  combustion  of  the  fine  refuse  anthracites,  or  of  tan,  bagasse 
and  the  like,  makes  this  method  of  draft  production  a  practical  necessity.  There- 
fore, the  economy  incident  to  the  utilization  of  such  fuels  must  be  considered 
as  a  direct  result  of  mechanical  draft.  Its  efficiency  is,  consequently,  to  be 
measured  by  the  percentage  of  saving  in  the  cost  of  fuel  for  a  given  evapo- 
ration, other  things  being  equal ;  and  this  saving  is  always  dependent  upon  the 
relative  costs  of  the  fuels  in  the  given  locality  and  the  expense  of  handling  them. 
But  it  is  evident,  from  the  results  shown  in  Chapter  V.,  that  large  savings  may 
be  assured  by  the  adoption  of  mechanical  draft  and  the  utilization  of  cheap  fuel 
in  any  locality  where  there  is  a  reasonable  opportunity  to  choose  between  the 
various  kinds  of  fuel. 


*  Steam  Boilers.     William  H.  Shock.     New  York,  1880. 

2  Steam-Boiler  Construction.     Walter  S.  Hutton.     London,  1891. 

3  Recent    Experience   with    Cylindrical    Boilers   and   the  "  Ellis   &    Eaves "  Suction    Draft. 
F.  Gross.     Transactions  Institution  of  Naval  Architects.     London,  1895. 


MECHANICAL   DRAFT.  235 

The  importance  of  mechanical  draft  in  the  combustion  of  cheap  fuels  is  thus 
indicated  by  Mr.  William  Parker:1  "Another  question  which  forced  draft  has 
satisfactorily  solved  is  the  use  of  small  and  inferior  coal.  There  is  at  least 
one  British,  and  there  are  three  Italian  steamers  now  running,  with  very  satis- 
factory results,  burning  nothing  but  very  inferior  coal,  which  could  not  possibly 
be  burned  with  natural  draft  with  the  ordinary  fire  bars." 

Economy  in  Quantity  of  Fuel. — There  may  be  conditions  under  which  a 
reduction  in  the  amount  of  fuel  consumed  may  be  of  greater  importance  than 
that  of  direct  decrease  in  the  total  cost  of  the  fuel,  which,  with  lower-priced 
fuel,  must  usually  be  used  in  greater  quantity.  The  question  becomes  most 
vital  in  its  relation  to  marine  practice.  For  short  voyages  a  decrease  in  the 
total  cost  of  the  fuel,  even  if  its  quantity  be  greater,  will  usually  result  in  a  net 
gain.  But  when  the  voyage  is  of  any  considerable  length  the  best  coal  is 
usually  found  to  be  most  economical ;  no  matter  what  the  fuel  may  be,  any 
reduction  in  its  quantity  resulting  from  the  use  of  mechanical  draft  must  add 
directly  to  the  freight-earning  dead-weight  capability  of  the  steamer.  Thus,  sup- 
pose that  for  a  steamer  of  2,500  tons  dead-weight  capacity  300  tons  of  coal  are 
required  for  an  Atlantic  passage,  with  ordinary  draft.  A  saving  of  20  per  cent 
of  fuel,  obtained  by  the  adoption  of  mechanical  draft,  would  mean  an  increase 
in  possible  weight  of  cargo  of  about  60  tons.  A  steamer  with  compound  en- 
gines and  mechanical  draft  would,  therefore,  be  on  a  par  with  one  having  triple 
expansion  engines  and  natural  draft. 

Incidental  to  any  reduction  in  the  amount  of  fuel,  whether  on  land  or  sea, 
is  the  direct  saving  in  the  cost  of  transportation,  handling  and  firing,  and  in  the 
lessened  amount  of  ashes.  Independent  of  its  character,  the  introduction  of 
economizers  or  heat  abstractors,  made  possible  by  the  adoption  of  mechanical 
draft,  presents  one  of  the  best  opportunities  for  reducing  the  quantity  of  fuel. 

Mechanical  Stokers.  —  As  stated  in  Chapter  VI.,  the  mechanical  stoker  is 
rendered  most  efficient  under  the  steadiness  and  intensity  of  draft  which  results 
from  the  substitution  of  a  fan  for  a  chimney.  Not  only  can  the  draft  be  easily 
regulated  to  meet  changes  in  the  rate  of  stoking,  but  it  is  capable  of  respond- 
ing even  more  quickly  than  the  stoker  to  a  demand  for  increased  evaporation. 
The  admission  of  air  through  hollow  grate  bars,  which  is  a  feature  of  certain 
mechanical  stokers,  can  only  be  successfully  attained  by  the  use  of  positive 
means  like  the  fan.  The  results  presented  in  Table  No.  82  demonstrate  the  pos- 
sible economy  with  a  combination  of  mechanical  draft  and  mechanical  stoking. 


i  On  the  Progress  and    Development  of    Marine  Engineering.     William  Parker.     Transac- 
tions Institution  of  Naval  Architects.     London,  1887. 


236 


MECHANICAL    DRAFT. 


Smoke  Prevention.  —  The  prevention  of  smoke  has  been  a  purpose  sufficient 
in  itself  to  often  warrant  the  introduction  of  a  fan  to  insure  rapid  combustion 
and  the  proper  supply  of  the  oxygen  necessary  to  accomplish  perfect  combustion. 
The  importance  of  the  fan  in  this  connection  has  already  been  shown  in  Chap- 
ter V.  Although  usually  a  mere  incident  to  its  application,  yet  the  prevention 
of  smoke  by  this  means  is  in  many  cases  one  of  the  most  important  advantages 
of  mechanical  draft,  to  be  measured  not  so  much  in  dollars  and  cents  as  in 
the  privilege  of  continuing  the  use  of  boilers  in  a  community  where  the  smoke- 
prevention  laws  are  enforced. 

Utilization  of  Waste  Heat  in  Gases. —  With  the  chimney  a  comparatively  high 
temperature  of  the  rejected  gases  is  an  absolute  necessity  to  the  production  of 
the  draft.  The  production  of  draft  by  means  of  a  fan  is,  on  the  other  hand, 
independent  of  the  temperature  of  the  gases,  and  there  is,  therefore,  presented 
the  opportunity  to  utilize  the  heat  which  is  a  positive  and  unavoidable  loss  in  the 
case  of  a  chimney.  How  great  this  loss  usually  is  has  already  been  shown  in 
Chapter  VI.,  and  the  importance  of  air  and  water  heaters  has  been  indicated. 
So  far  as  the  production  of  draft  is  concerned,  the  gases  may  be  cooled  down  to 
atmospheric  temperature,  but  the  practical  limit  is  necessarily  above  this  be- 
cause of  the  expense  of  the  abstracting  apparatus  required. 

The  saving  in  fuel  which  may  be  accomplished  under  working  conditions  by 
introducing  mechanical  draft  and  economizers  is  evidenced  in  Table  No.  120, 

Table  No.  120.  —  Results  of  Tests  of  Mechanical  Draft  Plants  and  Economizers. 


Temperatures. 

Plants 
Tested. 

i 

Fuel  Saving. 

Gases  Entering  j  Gases  Leaving 
Economizer.          Economizer. 

Water  Entering 
Economizer. 

Water  Leaving 
Economizer. 

Temperature  of 
Water. 

Per  cent. 

•   I 

I 

6lO°                      3400 

110° 

2870 

1770 

16.7 

2 

505                             212 

84 

276 

192 

I7.I 

3 

55° 

205 

I85 

3°  5 

120 

II-7 

4 

522 

320 

i.55 

300 

»45 

I3.8 

5 

.           5°5 

320 

190 

300 

no 

10-7 

6 

465 

250 

1  80 

295 

"5 

II.  2 

7 

490 

290 

165 

280 

"5 

II.O 

8 

495 

190 

J55 

320 

165 

'5-5 

9 

595 

299 

130 

3" 

181 

1  6.8 

i  Mechanical    Draft.     W.  R.  Roney.     Transactions    American    Society  of   Mechanical    En- 
gineers, Vol.  XV. 


MECHANICAL   DRAFT.  237 

which  presents  the  results1  of  tests  of  nine  plants,  in  nearly  all  of  which  the 
Sturtevant  fans  were  employed.  Under  all  ordinary  circumstances  an  econ- 
omizer can  be  relied  upon  to  bring  about  a  saving  of  from  10  to  20  per  cent. 

The  intensity  of  mechanical  draft  also  makes  possible  the'  introduction  of 
such  heat  abstractors  as  the  Serve  tubes  and  retarders,  the  resistances  pre- 
sented by  which  would  ordinarily  prevent  their  introduction. 

Economy  in  Space.  —  A  chimney  always  requires  specific  ground  area.  Owing 
to  its  weight  the  foundations  are  expensive,  and  where  the  ground  is  not  stable 
the  expense  may  be  almost  prohibitive.  The  fan  and  its  engine  are  compara- 
tively light  and  require  but  little  space  ;  and  even  that  may  usually  be  taken 
where  least  valuable,  as  overhead,  so  that  no  direct  charge  can  be  made  for 
ground  area  occupied.  This  is  of  great  importance  where  land  is  valuable,  and 
the  saving  may  go  far  towards  paying  for  the  apparatus. 

The  special  advantage  of  mechanical  draft  in  this  particular  has  already  been 
pointed  out  in  Chapter  VI.  and  illustrated  in  Figs,  i,  2  and  3,  where  it  was 
shown  that  this  item  might  become  of  considerable  importance. 

The  economizers,  or  heat  abstractors,  may  also  be  placed  overhead  within 
the  height  generally  allowed  for  a  boiler  house.  Even  the  size  and  cost  of  the 
boiler  house  itself  may  be  reduced. 

On  shipboard  the  matter  of  space  is  of  the  greatest  importance,  for  every  foot 
saved  leaves  just  so  much  more  for  coal  and  cargo.  In  a  discussion  of  this  mat- 
ter in  its  relation  to  the  equipment  of  a  transatlantic  liner,  Mr.  James  Howden,1 
with  his  system  of  forced  draft,  estimates  the  following  items  of  saving  and 
increased  return  (here  reduced  to  round  numbers  in  U.  S.  coinage)  result- 
ing from  the  decreased  space  in  a  given  ship  where  forced  draft  is  used :  — 

On  Round  Voyage. 

Cargo,  weight  and  measurement,  1,600  tons         .  .    $20,500 

1 60  first  and  second  class  passengers,  less  cost  of  food,     8,000 

80  third-class  passengers,  less  cost  of  food          .  .'         1,200 

Total  for  one  round  voyage         .....     $29,700 
Total  for  ten  round  voyages       .....  $297,000 

Economy  in  First  Cost.  —  As  between  the  greater  cost  of  a  chimney  and  its 
foundations,  and  that  of  a  fan  with  its  engine  and  foundations  to  fulfil  the  same 
requirements,  there  can  scarcely  be  a  question.  In  Chapter  VI.  it  was  shown 
that  the  cost  of  the  mechanical  draft  plant,  in  the  instance  quoted,  was  only  38 


i  On  Forced  Combustion  in  Furnaces  of  Steam  Boilers.     Transactions  Institution  of  Naval 
Architects.     London,  1886. 


238  MECHANICAL    DRAFT. 

per  cent  of  the  chimney  and  damper  regulators.  In  fact,  the  saving  there 
indicated  was  just  about  sufficient  to  cover  the  cost  of  the  economizers,  which 
might,  therefore,  have  been  installed  as  a  part  of  the  mechanical  draft  plant 
without  increasing  it  above  that  of  the  plant  with  chimney  and  without  econo- 
mizers. Consequently,  the  saving  due  to  the  introduction  of  the  economizers 
might  all  be  credited  to  mechanical  draft.  The  further  reduction  in  first  cost, 
resulting  from  decreased  size  of  boiler  plant  for  the  same  output  and  from 
reduction  of  space  occupied  and  building  required,  was  shown  to  be  sufficient  to 
make  the  aggregate  saving  about  three  and  a  half  times  the  amount  expended 
for  the  mechanical  draft  plant. 

The  following  editorial  comment,1  as  relating  to  marine  practice,  is  pertinent 
and  conclusive  :  "  There  appears  to  be  no  reason  why,  in  all  ordinary  marine 
boilers,  forced  draft  should  not  be  applied  with  such  good  results  as  to  insure, 
in  one  year,  the  recouping  of  the  capital  expended  upon  the  apparatus  and 
necessary  alterations." 

In  a  circular  on  the  comparative  cost,  efficiency  and  earning  power  of  a 
vessel  fitted  with  natural  draft,  and  one  equipped  with  a  forced-draft  system 
by  which  the  air  delivered  to  the  boilers  by  means  of  fans  was  specially  heated 
and  distributed,  Mr.  James  Howden  states  that  for  a  vessel  of  500  feet  length, 
57  feet  breadth  and  38  feet  depth,  "the  estimated  reduction  in  the  cost  of  the 
6  forced-draft  boilers  having  36  furnaces,  less  than  the  9  larger  natural-draft 
boilers,  with  the  cost  of  the  additional  mountings,  uptake,  funnel,  steam  and 
water  copper  pipes,  flooring,  boiler  room  and  extra  shipwork  required  for  the 
latter,  after  deducting  forced-draft  fittings,  is  ,£13,000  [about  $63,000]." 

Of  course,  in  all  cases,  whether  land  or  marine,  the  relative  cost  of  the  two 
methods  of  draft  production  must  depend  upon,  and  be  largely  affected  by,  the 
existing  circumstances.  But  the  conditions  must  be  exceptionally  unfavorable 
when  the  mechanical  draft  plant  cannot  show  decided  economy  in  first  cost 
when  compared  with  the  chimney. 

Decreased  Size  of  Boiler  Plant  for  Given  Output.  —  If,  as  shown,  the  output 
of  a  boiler  may  be  increased  by  the  adoption  of  mechanical  draft,  it  must  be 
conversely  true  that  its  size  may  be  reduced  and  the  initial  output  still  main- 
tained. In  stationary  practice  this  reduction  in  the  size  of  a  plant  necessary  to 
produce  a  given  result  decreases  both  the  floor  area  occupied  and  the  cost  of 
the  plant.  This  has  been  discussed  in  Chapter  VI.,  in  the  consideration  of  the 
influence  of  mechanical  draft  on  the  ultimate  efficiency  of  steam  boilers.  In 
marine  service  a  still  further  gain  results,  for  the  space  and  weight  saved  may 


Nautical  Magazine,  London,  Vol.  LVII.,  1888. 


MECHANICAL   DRAFT. 


239 


be  turned  to  direct  commercial  account  in  the  carrying  of  additional  freight  or 
fuel.  The  greater  the  amount  of  coal  which  can  be  carried  without  exceeding  a 
given  tonnage,  the  longer  the  voyage  the  vessel  can  make  at  a  given  speed  with- 
out replenishing  its  coal.  This  applies  particularly  to  naval  vessels  and  to 
steamers  with  which  quick  passages  are  to  be  made ;  for  it  is  apparent  that  the 
speed,  and  power  required  to  produce  the  same,  must  bear  such  relation  to  the 
coal  supply  and  the  distance  to  be  run  that  the  coal  shall  be  just  sufficient.  If 
it  is  sought  to  increase  the  efficiency  of  the  steam  plant  by  reducing  the  speed, 
this  may  be  offset  by  the  fact  that,  although  the  coal  expenditure  per  day  will 
be  less,  more  days  will  be  occupied  in  the  run,  and  the  total  result  may  be  prac- 
tically the  same  ;  while  in  addition  the  vessel  will  have  reduced  its  annual  earn- 
ing capacity. 

Mr.  Richard  Sennett1  distinctly  points  out  that  "the  many  advantages,  as 
regards  the  power  and  speed  of  steamships,  that  are  gained  by  this  reduction  in 
weight  and  space  required  for  the  boilers  are  too  obvious  to  require  enumera- 
tion ;  and  it  is,  perhaps,  not  too  much  to  say  that  the  very  high  speeds  that  have 
been  recently  obtained  by  several  cruisers  of  moderate  dimensions  would  have 
been  impracticable  without  the  application  of  forced  draft." 

The  greatly  increased  output  per  ton  of  boiler  with  mechanical  draft  is  most 
forcibly  presented  by  Table  No.  117,  wherein  the  average  indicated  horse-power 
shown  per  ton  of  boiler  is  11.98  with  natural  and  20.6  with  mechanical  draft; 
that  is,  the  total  weight  of  the  boilers  is  about  42  per  cent  less  under  the  latter 
condition. 

To  such  an  extent  as  the  dead-weight  cargo-carrying  capacity  of  merchant 
steamers  is  increased,  so  are  their  earnings  enhanced;  therefore,  any  means  by 
which  such  results  may  be  effectually  accomplished  demands  the  careful  consid- 
eration of  engineers.  Referring  to  the  Howden  system  of  mechanical  draft, 
Mr.  J.  W.  C.  Haldane2  says  :  "  For  very  many  years  it  was  considered  that  mod- 
erate combustion  in  good-sized  boilers  was  more  economical  than  forced  firing 
in  those  of  smaller  dimensions.  .  .  .  Leaving  theory  out  of  sight  and  turning 
to  actual  performance,  we  find  that  by  the  employment  of  this  system  the  boiler 
capacity  for  equal  powers  is  from  two-thirds  to  one-half  of  that  required  for 
natural  draft ;  while  its  use  secures  much  greater  economy  in  fuel  and  decreased 
wear  and  tear.  The  reduced  space  thus  required  for  boilers,  and  also  their  pro- 
portionately diminished  weight,  necessarily  make  this  system  vitally  important 
for  ocean  steamers." 


1  Closed    Stokeholds.      Richard    Sennett.      Transactions    Institution    of    Naval   Architects. 
London,  1886. 

2  Steamships  and  Their  Machinery.     J.  W.  C.  Haldane.     London  and  New  York,  1893. 


2 40  MECHANICAL    DRAFT. 

Economy  in  Operating  Expense.  —  In  fairness  to  both  chimney  draft  and 
mechanical  draft,  the  operating  expense  for  a  given  boiler  plant  should  compre- 
hend interest,  taxes,  rent,  insurance,  depreciation  and  repairs  upon  the  entire 
equipment,  as  well  as  fuel,  labor  and  transportation  charges,  per  unit  of  evap- 
oration. It  has  already  been  shown  that  under  all  ordinary  conditions  a  fan, 
with  its  engine,  connections  and  short  stack,  costs  less  than  a  chimney  to  accom- 
plish the  same  results ;  that  a  smaller  boiler  plant  may  with  mechanical  draft 
produce  the  same  output  as  one  of  larger  size  with  chimney  draft;  and  that  the 
cost  of  superficial  area  and  of  enclosing  building  will  therefore  be  less.  Hence 
the  fixed  charges  will  be  less.  If  an  economizer  be  introduced,  it  far  more  than 
pays  for  itself  in  the  saving  of  heat  accomplished.  The  actual  steam  ex- 
penditure for  operating  the  fan  will  not  in  a  plant  of  considerable  size  exceed 
i  per  cent  of  that  produced,  and  even  in  a  plant  of  small  size  will  seldom  if  ever 
exceed  3  to  4  per  cent.  Ordinarily  the  exhaust  thus  produced  will  be  utilized 
in  heating  feed  water  or  buildings,  and  thus  the  absolute  cost  of  operation  be 
reduced  to  an  infinitesimal  amount.  Even  if  this  steam  were  wastefully  thrown 
away,  the  economizer,  if  such  formed  a  part  of  the  plant,  would  compensate  for 
this  loss  in  addition  to  paying  its  own  fixed  charges.  Such  economy  of  fuel  per 
unit  of  evaporation  as  may  be  the  result  of  any  arrangement  of  mechanical 
draft  obviously  reduces  the  total  amount  of  fuel  consumed  and  lessens  the 
charges  for  labor  and  transportation.  As  a  rule,  any  saving  in  fuel  may  be  con- 
sidered as  a  net  gain,  whether  it  be  accomplished  by  an  improvement  in  the 
economy  of  combustion  of  that  previously  used  or  by  the  substitution  of  a 
cheaper  though  less  efficient  fuel. 

In  this  broad  sense  the  economy  of  operating  expense,  including  as  it  does 
all  contingent  items,  must  be  the  measure  of  commercial  efficiency.  The  respec- 
tive economies  secured  by  various  individual  features  of  mechanical  draft  have 
already  been  pointed  out  in  this  chapter.  Their  aggregate  effect  is  to  con- 
clusively prove  that  this  method  of  draft  production  possesses  economic  advan- 
tages, not  to  mention  immeasurable  conveniences,  which  make  it  indispensable 
to  the  model  steam  plant. 

Ventilation.  —  In  stationary  boiler  plants  the  effect  of  mechanical  draft  upon 
the  ventilation  of  the  boiler  room  is  of  minor  importance,  but  upon  shipboard 
it  is  almost  vital.  Under  the  latter  condition  the  furnishing  of  large  volumes  of 
air  not  only  keeps  fresh  that  within  the  fire  rooms,  but  coincidently  lowers  its 
temperature,  which  is  of  almost  as  much  importance.  When  the  overpowering 
conditions  of  the  ordinary  marine  fire  room  are  considered,  it  is  evident  that 
such  provision  of  fresh  air  as  may  result  from  mechanical  draft  is  twofold  in 
its  effect.  From  a  humanitarian  standpoint,  the  men  are  kept  in  better  physical 


MECHANICAL   DRAFT.  241 

condition,  while  in  its  mercenary  aspect  the  results  are  directly  evident  in  in- 
creased efficiency  of  service.  As  stated  by  Mr.  Richard  Sennett,1  "when  the 
system  was  first  adopted  it  was  suggested  that  the  men  might  have  some  reluc- 
tance to  work  in  closed  stokeholds.  This,  however,  has  not  proved  to  be  the 
case,  and  the  men  have,  from  the  first,  worked  as  confidently  and  cheerfully  as 
in  ordinary  stokeholds.  The  only  effect  that  the  closing  of  the  stokeholds  has 
had  upon  the  men  has  been  to  enable  them  to  do  their  work  in  more  comfort 
in  consequence  of  the  better  ventilation.'' 

Summary  of  Advantages.  —  In  the  preceding  discussion  of  the  advantages  of 
mechanical  draft,  the  difficulty  of  presenting  each  as  independent  of  the  others 
must  have  been  evident.  To  a  great  extent  they  are  interdependent,  and  the 
possession  of  one  advantage  is  evidence  of  the  possession  of  others  of  similar 
character.  In  a  brief  summary,  however,  these  may  be  more  readily  brought 
into  accord.  Thus  the  very  adaptability  of  mechanical  draft  is  indicative  of  the 
fact  that  it  is  more  flexible  than  that  produced  by  the  chimney,  is  more  readily 
controlled,  and  less  influenced  by  climatic  changes ;  while  the  apparatus  for  its 
production  is  more  readily  transported  and  has  a  higher  potential  value  than  a 
chimney.  To  a  considerable  extent  these  stand  out  as  the  conveniences  of  this 
method,  regardless  of  their  economies.  When  it  is  shown  that  increased  effi- 
ciency can  be  secured  by  a  method  that  is  more  convenient,  the  advantage  of 
mechanical  draft  is  established. 

The  actual  omission  of  the  chimney  is  sometimes  of  far  greater  importance 
than  would  at  first  appear,  while  the  readiness  with  which  the  rate  of  combustion 
may  be  increased  is  doubly  appreciated  when  it  is  shown  that  under  proper  con- 
ditions the  efficiency  of  combustion  may  be  increased  thereby.  The  purely 
economic  features  are  presented  most  prominently  in  the  ability  to  utilize  low- 
grade  fuels,  the  resultant  economy  being  shown  in  numerous  examples  here  pre- 
sented. The  economy  in  the  quantity  of  fuel  consumed  has,  in  its  relation  to 
the  use  of  mechanical  draft  on  shipboard,  an  advantage  which  is  closely  allied 
to  that  resulting  from  the  decreased  space  occupied. 

The  economic  results  which  may  be  secured  through  the  introduction  of 
mechanical  stokers  and  devices  for  utilizing  the  waste  heat  of  the  gases  are 
rendered  most  evident  under  the  conditions  of  mechanical  draft  production,  as 
are  also  the  great  advantage  of  preventing  smoke  and  the  blessings  of  good 
ventilation  as  they  are  exemplified  on  shipboard.  The  facts  that  the  size  of  a 
boiler  plant  required  for  a  given  output  can  be  reduced  when  a  fan  is  substituted 


'  Closed    Stokeholds.      Richard    Sennett.      Transactions    Institution    of   Naval    Architects. 
London,  1886. 


2  42  MECHANICAL    DRAFT. 

for  a  chimney,  that  the  cost  of  the  mechanical  draft  plant  is  usually  far  less 
than  that  of  the  chimney  draft  plant,  and  that  its  operating  expense  is  likewise 
less  under  proper  conditions,  all  point  most  conclusively  to  the  purely  economic 
advantages  of  the  method  which  it  is  the  purpose  of  this  book  to  present. 
When  these  are  considered  in  the  light  of  the  convenience  and  various  other 
advantages  of  mechanical  draft,  its  evident  superiority  to  chimney  draft  must 
be  conclusively  established  in  the  mind  of  any  one  who  has  read  these  pages. 


CHAPTER    XII. 
THE    STURTEVANT  FANS   FOR   MECHANICAL   DRAFT. 

It  has  been  made  evident  in  the  preceding  chapters  that  the  essential  feature 
of  mechanical  draft  is  a  fan  blower  or  exhauster.  It  has  been  shown  that  the 
blowing  engine  and  the  positive  rotary  blast  blower  are  not  adaptable,  that  the 
steam  or  compressed-air  jet  is  not  economical,  and  that  the  disc  or  propeller 
form  of  fan  wheel  is  not  suitable  for  the  purpose.  The  peripheral  discharge 
type  of  fan  therefore  stands  as  the  only  form  which  it  is  desirable  to  employ 
for  the  production  of  draft.  It  is  the  purpose  of  this  chapter  to  illustrate  and 
describe  this  type  of  fan  in  all  of  its  principal  forms  as  built  by  this  house. 
It  is,  however,  manifestly  impossible  to  present  all  of  the  multitudinous  shapes 
in  which  these  fans  are  constructed  to  suit  the  ever-varying  requirements  of 
different  plants. 

Prominent  among  the  advantages  of  mechanical  draft  as  displayed  in  the 
preceding  chapter  is  that  of  adaptability,  as  is  most  clearly  evidenced  in  this 
and  the  succeeding  chapter.  The  steel-plate  construction  employed  in  all  fans 
but  those  of  smaller  size  lends  itself  most  readily  to  perfect  adaptation  to  the 
conditions  existing  in  any  specific  case.  The  fan  may,  if  absolutely  necessary, 
be  small  and  be  operated  at  high  speed,  or,  as  should  otherwise  be  the  case,  it 
may  be  large  and  run  slowly.  It  may  be  constructed  of  steel  plate  in  all  sizes, 
and  of  cast  iron  in  the  different  types  of  the  smaller  sizes.  In  the  former 
material  it  may  take  almost  any  shape  within  the  range  of  possible  require- 
ments, while  either  cast-iron  or  steel-plate  fans  are  regularly  constructed  to 
discharge  either  horizontally  at  the  top  or  bottom,  or  directly  upward  or  down- 
ward. The  pulley  or  engine,  according  as  one  or  the  other  is  employed,  may 
be  placed  upon  either  side  of  the  fan ;  while  the  engine  may,  to  suit  circum- 
stances, be  single  or  double,  open  or  enclosed,  with  its  cylinders  above  or  below 
the  shaft,  or  may  be  horizontal  and  of  any  required  size.  Or  if  desired  a  direct- 
connected  electric  motor  may  take  the  place  of  the  engine  in  all  but  the  largest 
sizes.  The  most  important  of  these  various  arrangements  are  presented  in  the 
succeeding  illustrations,  while  in  Chapter  XIII.  are  shown  specific  applications, 
in  many  of  which  the  particular  conditions  demanded  the  construction  and 
introduction  of  the  particular  forms  of  fans  which  there  appear. 


244 


MECHANICAL    DRAFT. 


Steel  Pressure  Blower.  — 'The  type  of  fan  presented  in  Fig.  15  is,  as  its  name 
indicates,  a  pressure  blower  rather  than  a  volume  blower.  That  is,  the  wheel 
is  of  such  dimensions  as  to  make  it  possible  to  deliver  a  comparatively  small 
amount  of  air  under  high  pressure.  This  is  the  requirement  in  the  case  of 
cupola  furnaces  and  forges  for  which  this  type 
was  originally  designed.  Substantially 
same  requirement  exists  in  the  case  of 
some  of  the  under-feed  mechanical 
stokers,  where  a  very  deep  bed  of 
fuel  is  maintained  and  consider- 
able pressure  is  necessary  to  over- 
come its  resistances,  although  the 
actual  volume  of  air  required  is 
not  great.  Under  similar  condi 
tions  it  is  useful  in  connection 
with  crematories,  garbage  de- 
structors and  the  like.  These 
fans  are  capable  of  producing  a 
pressure  as  high  as  20  ounces 
per  square  inch.  The 
fan  wheel  consists  of 
a  light  but  strong 
hub  with  extending 
arms  and  a  series  of 
galvanized  steel- 
plate  blades  or 
floats  attached 
thereto.  Conical 
side  plates  are 
attached  to  these 

blades     and     ex-  FIG> 

tend  from  inlet  to 

circumference.  The  steel  shaft,  to  which  the  hub  is  keyed,  is  supported  upon 
either  side  by  special  continuous-oiling  journal  boxes,  which  are  of  such  length 
and  so  thoroughly  oiled  that  heating  is  practically  impossible.  The  smaller 
fans  are  each  provided  with  a  single  pulley,  placed  between  the  box  and  the  fan 
case  upon  the  right-hand  side  as  one  faces  the  outlet,  the  fan  being  then  known 
as  a  right-hand  fan.  The  larger-sized  fans  are  each  provided  with  two  pulleys, 
one  upon  either  side. 


STEEL  PRESSURE  BLOWER. 


MECHANICAL    DRAFT. 


245 


"Monogram"  Blower.  —  The  "  Monogram"  Blower,  illustrated  in  Fig.  16,  and 
so  designated  because  of  the  makers'  monogram  upon  its  side,  is  similar  in  gen- 
eral design  to  the  steel  pressure  blower  just  described.  The  journal-box  and 
shaft  construction  is  the  same,  but  the  wheel  is  wider,  being  designed  for  the 
handling  of  a  considerable  volume  of  air ;  while  the  outlet  for  a  given  height  of 
shell  is  larger,  in  order  to  accommodate  the  greater  volume.  Fans  of  this  type  are 
provided  with  only  one  pulley,  which  may  be  placed  upon  either  the  right  or  left 
hand  side  as  one  faces  the  outlet,  thereby  making  the  blower  respectively  right 
or  left  hand.  The  construction,  flErf'^baj-  which  insures  rigidity,  is 
conducive  to  continuous  opera-  JIJr:  ti°n  at  high  speed,  with- 

out any  disagreeable  noise  or  Mmm.  ^SSBBIfiS^.  inconvenience  from 
heating. 

These    fans  are    particu- 
the    production    of   forced 
tively  small  boiler  plants, 
such  pressures  as  may  be 
dling  furnaces  and  forges, 
applied  for  boiler  draft, 
at  a  point  where  it 
drive  by  belt,  and 
may  be  led  to  the 
this   is    the   type 
troduced  by  this 
third  of  a  century 
draft    to    steam 
of   its    ability   to 
erable    pressure 

come  of  particular  value  in  the  production  of  draft  for  the  burning  of  bagasse. 
It  is  also  applicable  in  many  cases  where  the  steel  pressure  blower  could  other- 
wise be  introduced  and  operated  at  moderate  speed.  This  is  true  of  its  appli- 
cation for  some  types  of  mechanical  stokers,  where  a  pressure  is  desired 
somewhat  in  excess1  of  that  which  it  would  be  advantageous  to  maintain  by  a 
steel-plate  fan.  One  of  the  extensive  applications  of  the  smaller  sizes  of  these 
fans  is  for  producing  the  blast  required  with  the  various  forms  of  hollow  grate 
bars,  from  which  the  air  is  discharged  in  minute  streams  directly  into  the  bed  of 
fuel.  Exhausters  of  this  type,  having  an  inlet  upon  the  side  only,  with  both 
bearings  and  pulley  outside  the  case,  upon  the  other  side,  may  be  arranged 
•with  special  cooling  devices,  so  as  to  produce  draft  by  the  induced  system, 
the  gases  being  passed  through  the  fan. 


FIG.  16.    "MONOGRAM"  BLOWER. 


larly  adapted  for 
draft  in  compara- 
and  for  creating 
required  for  pud- 
The  blower,  as 
may  be  located 
is  convenient  to 
thence  the  airpipe 
ashpit.  In  fact, 
of  fan  first  in- 
Company,  over  a 
ago,  for  furnishing 
boilers.  Because 
maintain  consid- 
this  type  has  be- 


246 


MECHANICAL   DRAFT. 


"Monogram"  Blower  on  Adjustable  Bed.  —  It  is  particularly  important,  in 
the  case  of  a  blower  employed  for  draft  production,  that  there  should  be  no 
liability  of  its  stoppage  during  working  hours.  So  far  as  the  construction  of  the 
Sturtevant  blower  is  concerned,  this  is  obviated  by  the  character  of  the  design 
and  the  perfection  of  the  construction.  But  when  driven  by  belt  there  is  always 
a  possibility  of  the  tension  thereon  gradually  decreasing  until  it  suddenly  be- 
comes apparent  in  the  slowing-down  of  the 
blower.  To  shut  down  long  enough  to  take 
up  the  slack  is  a  great  inconvenience 
when  boilers  are  depending  upon  the 
blower  for  the  production  of  their 
draft.  To  avoid  this  necessity,  the 
arrangement  illustrated  in  Fig.  17 
can  be  furnished.  As  is  evident  from 
the  cut,  the  blower  is  placed  upon  a 
bed  upon  which  it  is  adjustable,  so 
that  the  belt  may  be 
continually  kept  tight. 
In  its  construction 
the  bed  cqnsists  of 
substantial  steel  side 
beams  which  are  rig- 
idly connected  at 
their  ends  by 
castings,  to 
which  they 
are  bolted. 
The  blower  it- 
self is  clamped 
to  the  beams  by 

bolts  passing  down  through  its  feet.  The  combination  of  beams  and  bolts 
serves  to  guide  the  blower  and  keep  the  belt  aligned  as  it  is  drawn  forward 
by  means  of  the  shackle  bolt,  which  is  attached  to  the  front  of  the  blower, 
just  beneath  the  outlet,  and  passes  through  the  front  casting.  To  avoid  in- 
terference with  any  connecting  outlet  pipe,  resulting  from  the  movement  of 
the  blower,  a  telescopic  outlet  is  provided  which  is  bolted  to  the  end  casting 
of  the  bed  and  within  which  a  sheet-iron  extension  of  the  outlet  of  the  blower 
slides  as  it  is  moved.  The  steel  pressure-blower  type  of  fan  may  be  fitted  up  in 
the  same  way.  The  entire  combination  is  readily  portable. 


FIG.  17.     "MONOGRAM"  BLOWER  ON  ADJUSTABLE  BED. 


MECHANICAL    DRAFT. 


247 


" Monogram"  Blower  on  Adjustable  Bed  with  Engine.  —  The  arrangement 
just  illustrated  and  described  depends  for  its  propulsion  upon  some  means  inde- 
pendent of  the  blower  itself.  But  in  such  an  important  matter  as  the  constant 
maintenance  of  draft  it  is  particularly  desirable  that  the  blower  should  be  pro- 
vided with  such  means  of  operation  as  to  render  it  entirely  independent  of  any 
other  source  of  power.  In  the  case  of  a  blower  of  the  type  under  discussion, 
an  engine  may  be  readily  combined  with  it  upon  the  same  bed.  Such  is  the 


arrangement  shown  in  Fig.  18,  where  the  en- 

cylindered,  entirely  enclosed. 

maintaining  high   speed, 

perfect    regulation    by 

ernor.     The  enclosed 

engine   prevents  the 

throwing  of  oil  and 

avoids  the  danger  of 

injury  to  the  bearings 

from  the  flying  dust 

and  dirt, 

almost 

always 

present 


gine  is  double- 
is  capable  of 
and  subject  to 
the  shaft  gov- 
feature  of  the 


FIG.  18.    "MONOGRAM"  BLOWER  ON  ADJUSTABLE  BED  WITH  COMBINED  DOUBLE 
ENCLOSED  UPRIGHT  ENGINE. 

in  a  boiler  room.  The  same  arrangement  can  be  furnished  with  a  single  up- 
right or  a  horizontal  engine  in  place  of  the  engine  here  shown.  Any  such  com- 
bination makes  it  possible  to  start  up  the  boilers  independently  of  any  other 
portion  of  the  steam  plant. 

The  bed  proper  is  of  the  same  general  construction  as  that  described  in  con- 
nection with  Fig.  17.  In  case  an  engine  is  not  desired,  a  counter-shaft  (with 
tight  and  loose  pulleys,  if  required)  can  be  substituted,  and  the  adjustable 
feature  still  retained.  Evidently  a  pressure  blower  may  be  as  readily  fitted  up 
in  any  of  these  various  combinations. 


248 


MECHANICAL    DRAFT. 


FIG.  19.     STEEL-PLATE  BLOWER  WITH  OVERHUNG  PULLEY. 


MECHANICAL   DRAFT. 


249 


Steel-Plate  Blower.  —  For  moving  large  volumes  of  air  under  moderate  pres- 
sure, the  type  of  fan  illustrated  in  Fig.  19  is  extensively  employed.  The  shell 
is  constructed  throughout  of  steel  plate  supported  upon  an  angle-iron  foundation 
frame  and  braced  and  stiffened  by  the  same  material.  The  entire  construction 
is  relatively  light  but  strong,  and  may  obviously  be  made  to  conform  to  any  de- 
sired requirements.  As  here  shown,  the  blower  has  a  bottom  horizontal  dis- 
charge. This  type  is  also  regularly  built  to  discharge  horizontally  at  the  top  or 
directly  upward  or  downward. 

An  inlet  is  provided  in  each  side  of  the  shell.  A  fan  thus  provided  is  des- 
ignated as  a  blower,  while  one  having  only  a  single  inlet  (which  is  placed  on  the 
side  farthest  from  the  pulley)  is  known  as  an  exhauster.  This  blower  has  a 
bearing  in  each  inlet,  with  the  fan  wheel  between,  and  the  pulley  overhung  on  the 
end  of  the  shaft.  The  minimum  of  width  is  thus  occupied,  and  this  type  of  fan 
is  thus  rendered  convenient  for  most  applications  for  forced  draft.  In  the  larger 
sizes  it  is  so  constructed  that  the  entire  top  may  be  readily  taken  off  to  obviate 
the  objection  to  excessive  height  under  the  conditions  of  railroad  transportation, 
which  permit  of  only  a  certain  maximum  height. 

The  fan  wheel  which  is  enclosed  within 
the  shell  is  of  the  form  illustrated 
in  Fig.    20.     It   consists  of   a 
series  of  T-steel  arms  cast  into 
the  hubs  —  of  which  there  are 
two  —  carrying  the  floats  or 
blades,  which,  together  with 
the    side    plates  of 
the  wheel,  are  con- 
structed   of    steel 
plate.      The    fan 
wheel  is  carefully 
balanced  to  insure 
its  steady  running. 
The   journal   boxes, 
here  clearly  shown,  are  of 
the  Sturtevant  patent  brush- 
oiler   pattern,    the    oil   being 
continuously  fed  to  the  bearing 
by  means  of  a  brush  submerged 
in  a  reservoir  which  remains  filled 
FIG.  20.     FAN  WHEEL.  to  a  certain  level. 


250 


MECHANICAL    DRAFT. 


Steel-Plate  Blower  on  Adjustable  Bed  with  Engine.  —  The  adjustable  ar- 
rangement which  has  already  been  illustrated  in  connection  with  the  pressure 
and  "Monogram"  blowers  is  also  applicable  to  the  steel-plate  blower,  as  is 
rendered  evident  in  Fig.  21.  A  single  engine  is  here  shown,  but  a  double 
engine  or  a  counter-shaft  could  as  readily  form  a  part  of  the  combination.  The 
necessity  of  a  belted  engine  is  due  to  the  high  rotative  speed  of  the  fan,  which 
would  be  excessive  for  a  direct-connected  engine  of  proper 
power.  The  utility  of  such  an  arrangement  is  obvious.  It 
is  readily  portable,  may  be  set  up  wherever  desired  without 
the  preparation  of  special  foundations,  is  not  dependent  for 


FIG.  21. 


STEEL-PLATE  BLOWER  ON  ADJUSTABLE  BED  WITH  COMBINED  SINGLE 
UPRIGHT  ENGINE. 


its  operation  upon  any  other  source  of  power,  and  may  be  so  regulated  that  the 
speed  of  the  engine  shall  increase  as  the  steam  pressure  falls.  By  this  latter 
combination  the  range  of  variation  in  steam  pressure  may  be  reduced  to  a  min- 
imum—  in  fact,  kept  within  one  or  two  pounds. 

Evidently  such  an  arrangement  is  suitable  only  for  the  production  of  draft  by 
the  forced  method,  but  the  entire  equipment  may,  if  desired,  be  placed  on  top  of 
the  boilers  and  the  use  of  valuable  floor  space  avoided  ;  or  it  may,  at  the  expense 
of  comparatively  little  room,  be  placed  along  one  side  of  the  end  boiler  of  a  bat- 
tery and  discharge  into  an  underground  duct  beneath  or  in  front  of  the  ashpits. 


MECHANICAL    DRAFT. 


Steel-Plate  Exhauster.  —  As  already  stated,  the  distinguishing  feature  of  an 
exhauster  is  the  single  inlet,  placed  in  the  side  farthest  removed  from  the  pulley 
or  other  means  of  propulsion.  The  standard  form  of  steel-plate  exhauster  is 
shown  in  Fig.  22.  This  form  of  construction  makes  possible  the  ready  connection 
of  a  pipe  to  this  inlet  for  the  purpose  of  exhausting  air  or  gas  from  any  particu- 
lar space.  The  wheel  being  overhung  upon  the  end  of  the  shaft,  and  the  pulley 
and  boxes  all  being  located  upon 
the  same  side  of  the  fan 
housing,  the  inlet  is  left  en- 
tirely unobstructed  and 
there  is  no  opportunity 
for  injury  to  the  bear- 
ings by  dust  or  heat. 
This  type  of  fan  is 
equally  adaptable  for 
use  as  a  blower,  all 
of  the  air  then  being 
taken  in  on  one  side. 
For  the  purpose  of  in- 
duced draft  it  is  by  far 
the  best  form,  for 
a  special  Sturte-  / 
vant  water-cooled 
journal  box  may 
be  easily  substi- 
tuted for  the 
inner  bearing 
and  the  trans- 
mission of  heat 
along  the  shaft 
thereby  pre- 
vented. If  the  air  or  gas  handled  is  of  excessively  high  temperature,  the  support 
may  be  set  away  from  the  shell  by  spacing  pieces  so  as  to  allow  a  circulation  of 
air  between.  This  support,  which  may  be  rigidly  bolted  to  the  floor  or  founda- 
tion, carries  the  entire  weight  of  the  boxes,  shaft,  pulley  and  wheel,  thereby 
removing  from  the  fan  casing  all  strain  due  to  weight  or  tension  of  the  belt 
The  casing  itself  is  of  substantial  steel  plate,  except  the  outlet  frame  and  bot- 
tom plate,  which  are  of  cast  iron  ;  while  the  wheel  is  of  the  same  construction 
as  that  which  has  already  been  illustrated. 


G.  22.     STEEL-PLATE  EXHAUSTER  WITH  OVERHUNG  WHEEL. 


252 


MECHANICAL   DRAFT. 


Steel-Plate  Steam  Fan.  —  It  is  always  desirable  that  the  means  of  propulsion 
for  a  fan  should  be  as  independent  as  possible  of  any  other  source  of  power;  in 
other  words,  the  motor  adopted  should  be  devoted  solely  to  the  driving  of  the 
fan.  In  the  smaller  sizes  of  fans  of  the  pressure  and  "Monogram"  types,  the 
speed  of  rotation  to  produce  the  required  pressure  is  such  that  a  motor  in  the 
form  of  a  steam  engine  directly  connected  to  the  fan 
shaft  would  be  obliged  to  operate  at  too  high  a  speed 
to  remain  durable;  hence  the  belted  arrangements 
which  have  already  been  shown.  In  the 
larger  sizes  of  fans,  however,  partic- 
ularly those  of  steel  plate,  the  speed 
is  such  as  to  make  direct  con- 
nection practicable.  A  common 
form  of  this  arrangement  is 
that  illustrated  in  P'ig.  23. 
The  fan  itself  is  an  exhauster, 
being  identical  in  form  and 
construction  with  that  shown 
in  Fig.  22,  with  the  exception 
that  the  shape  of  the  support 
is  changed  and  that  an  en- 
gine is  substituted  for  the 
journal  boxes  and  pulley. 
This  form  of  engine,  which 
has  its  cylinder  above  the 
shaft,  is  of  the  same 
construction  as  the  reg- 
ular automatic  upright 
engines  built  by  this 
Company.  The  valve 
is  of  the  balanced  pis- 
ton type,  the  cylinder 
is  thoroughly  lagged, 
the  crank  is  accurately  counter-balanced,  and  the  crank  pin  is  oiled  from  a  sta- 
tionary sight  feed  oiler,  attached  to  the  frame  of  the  engine.  Large-cylindered 
low-pressure  engines  can  be  furnished  in  this  type.  Evidently  this  construction 
readily  lends  itself  to  application  for  mechanical  draft,  particularly  under  the 
induced  system;  for  the  wheel  is  overhung  upon  the  end  of  the  shaft  and  the 
inner  journal  may  be  water  cooled. 


FIG.  23.     STEEL-PLATE  STEAM  FAN  WITH  ENGINE  HAVING 
CYLINDER  ABOVE  THE  SHAFT. 


MECHANICAL   DRAFT. 


253 


FIG.  24.      STEEL-PLATE  STEAM  FAN  WITH  ENGINE  HAVING  CYLINDER  BENEATH  THE  SHAFT. 


254 


MECHANICAL    DRAFT. 


The  form  of  steam  fan  illustrated  in  Fig.  24  is  that  employed  in  the  larger 
sizes  of  full-housing  steel-plate  steam  fans.  As  is  evident,  it  is  specially  con- 
structed for  this  particular  use,  its  cylinder  is  beneath  the  shaft,  and  it  possesses 
but  a  single  bearing,  the  other  bearing  for  the  shaft  being  regularly  placed  upon 
a  truss  in  the  inlet  upon  the  opposite  side  of  the  fan.  When  applied  for  in- 
duced draft,  the  shaft  may  be  extended  so  that  its  supporting  journal  box  can  be 
placed  outside  the  inlet  connection.  Both  this  bearing  and  that  upon  the  en- 

me  may  be  chambered  and  kept  cool 
by  a  constant  circulation  of  water. 
The  space  which  in  the  usual  con- 
struction is  left  between  the 
engine  and  the  shell  ob- 
viates any  further  trouble 
i  from  direct  transmission 
to  the  engine.     Various 
applications  of  this  and 
the    previously    illus- 
trated form  of    steam 
fan  will  appear  in  the 
succeeding    chapter. 
Both   forms    lend  them- 
selves to  control  by  auto- 
matic draft  regulators, 
which  may  be  so  ar- 
ranged that  as  the 
steam     pressure 
falls  the  engine 
speed  and   conse- 
quently  the   draft 
pressure  and  rate 
of  combustion  rise 
and  more  steam  is 
at  once  generated. 

Steel-Plate  Steam  Fan  with  Engine  Enclosed.  —  The  objection  to  the  pres- 
ence of  an  engine  in  some  boiler  rooms  is  usually  that  of  liability  to  damage 
from  the  fine  dust  which  is  floating  in  the  atmosphere  and  constantly  tends  to 
work  into  the  bearings  with  disastrous  effect.  This  objection  may  be  removed 
by  entirely  enclosing  the  engine  in  a  steel-plate  casing  as  shown  in  Fig.  25.  A 
regular  form  of  double  enclosed  engine  is  shown  in  subsequent  cuts. 


FIG.  25.     STEEL-PLATE  STEAM  FAN  WITH  ENGINE  ENCLOSED. 


MECHANICAL    DRAFT. 


255 


Steel-Plate  Exhauster  with  Inlet  Connection.  —  When  an  exhaust  fan  is  to  be 
employed  for  induced  draft  it  is  frequently  desirable  to  construct,  in  connection 
with  and  in  fact  as  a  part  of  the  fan,  an  inlet  connection  in  the  manner  indicated 
in  Fig.  26.  As  there  shown,  with  the  shaft  extended  through  the  connection  and 
supported  by  an  outside  journal  box,  the  arrangement  is  particularly  adaptable 
to  any  type  of  fan,  whether  steam  or  pulley,  such  as  is  shown  in  Figs.  19  and  24, 
in  both  of  which  the  shaft  is  ordinarily  supported  by  a  bearing  in  the  inlet. 
Naturally  the  external  bearings  would 
be  provided  with  cooling 


FIG.  26.     STEEL-PLATE  EXHAUSTER  WITH  INLET  CONNECTION. 

devices  if  hot  air  or  gas  is  to  be  handled,  and  therefore  thereby  rendered  per- 
fectly serviceable  even  under  these  somewhat  trying  conditions. 

As  here  represented  the  inlet  connection  is  of  steel  plate,  with  angle-iron 
corner  frames  and  additional  bracing  of  heavy  angle  iron.  It  is  provided  with 
a  door  to  permit  of  access  to  the  interior  of  the  connection  and  of  the  fan  for 
the  removal  of.  soot  and  dust.  Although  shown  with  open  bottom  for  the  ad- 
mission of  air  or  gas,  it  may  as  readily  be  constructed  so  that  the  supply  can  be 
taken  from  above  or  through  either  side.  The  bottom  connection  is  especially 
desirable  if  the  fan  is  to  be  placed  above  the  boilers  and  the  gas  taken  from  a 
flue  beneath. 


256 


MECHANICAL   DRAFT. 


Special  Steel-Plate  Steam  Fans.  —  The  types  of  independent  fans  which  have 
thus  far  been  presented  are  those  of  regular  form.  But,  in  the  adaptation  of  fans 
for  mechanical  draft,  many  special  forms  are  required,  particularly  for  application 
on  shipboard.  These  are  generally  provided  with  independent  engines,  in  each 
case  directly  connected  to  the  fan  shaft.  Great  variety  in  the  character,  form 
and  proportions  of  these  engines  is  necessary  to  make  them  readily  adaptable ; 
as  a  consequence,  the  differences  between  most  of  the  fans,  the  illustrations  of 
which  here  follow,  lie  fully  as  much  in  the  engines,  by  means  of  which  they  are 
driven,  as  in  the  fans  themselves. 

The  smallest  and  simplest 


by  this  Company  is  shown 
construction  of  the  shell 
the  regular  steel-plate 
gine  is  self  contained, 
upon     its    extended 
signed  for  operation 
is  provided  with  sight- 
adjustable  in  all  im- 
This  size  and  type  is 
forced -draft   produc- 
steam  yachts  where 
is  limited  in  area, 
the    creation    of 
sure.     Another 
sively   employed 
general     marine 
sented  in  Fig.  28. 
tionof  the  fan  and 
of  channel  beams 
the   shell  of   the 


SPECIAL  STEEL-PLATE  STEAM  FAN, 
WITH  SINGLE  ENGINE. 


of  this  class  manufactured 
in  Fig.  27.    The  general 
is  the  same  as  that  of 
jk    exhausters.    The  en- 
Ik    carries  the  fan  wheel 
Ift   shaft,   and  is   de- 
Jfjl    at  high  speed.     It 
feed  oilers  and  is 
portant  bearings, 
serviceable    for 
tion     on     small 
the  grate  service 
being    used    for 
under-grate  pres- 
type,  also  exten- 
for     yacht     and 
work,    is    repre- 
Here  the  founda- 
engine    consists 
extending    from 


vessel  to  an  in- 
terior support,  thus  bringing  the  top  of  the  fan  casing  close  up  to  the  deck. 
Owing  to  the  limited  space  in  the  steam  yacht  Sapphire,  for  which  this  was  de- 
signed, the  outlet  was  formed  in  the  side  of  the  casing,  the  air  being  deflected 
thereto  by  a  curved  plate  within  the  casing.  From  this  outlet  a  pipe  leads  to 
the  boiler  ashpit.  Evidently  this  arrangement  occupies  the  minimum  of  space. 
The  engine  is  of  the  double-cylindered  type  subsequently  illustrated  in  Fig.  32. 
It  is  particularly  adapted  for  this  location  because  of  its  compactness,  its  per- 
fect balance,  its  ability  to  run  at  high  speed  for  a  long  period  and  its  enclosure 
from  dust  and  dirt. 


MECHANICAL    DRAFT. 


257 


FIG.  28.     SPECIAL  STEEL-PLATE  STEAM  FAN  WITH 
DOUBLE  ENCLOSED  ENGINE. 


It  is  sometimes  the  case 
that  an  upright  engine  of  the 
type  just  described  will,  if 
of  adequate  power,  require 
a  greater  height  than  the 
conditions  will  admit.  This 
difficulty  may  in  some  cases 
be  avoided  by  using  an  en- 
gine of  the  same  type  with 
the  cylinders  below  the  shaft, 
as  shown  in  a  succeeding 
illustration;  but  when  neither 
form  is  admissible  resort 
must  be  had  to  a  special  type 
of  horizontal  engine.  This 
was  the  condition  which 
held  in  the  design  of  the 
special  fan  shown  in  Fig.  29, 

which  represents  one  of  several  fans  constructed  for  U.  S.  S.  Monadnock. 
The   engine   is   self-contained,  having  two  bearings;    and  the  fan 

wheel  is  supported  on  the  end  of  the  shaft. 

The  crank   and  connecting 

mechanism  are  entirely  en- 
closed, preventing  the  throwing 

of  oil  and  the  admission  of  dust.  |1 

By  the  combined  effect  of   the 

cast-iron  bracket    and    the    angle- 
iron    sling,    the    engine    is    held 

rigidly  in  its  place.     Being  carried 

close  up  to  the  deck,  the  fan  being 

in  fact  fitted  in  between  the  deck 

beams,  the    least    possible    head 

room  is  occupied.     Evidently  such 

a  fan  can  be  arranged  to  deliver  in 

any  given  direction  or  entirely  around 

the  circumference,  as  might  be  desirable 

in  a  dosed  fire  room.     Other  forms  for 

use  in\  marine  work  are    presented  in       Fl(;   2g     Sp£CIAL  STEEL.PLATE  STEAM 

the  next  chapter.  FAN  WITH  HORIZONTAL  ENGINE. 


MECHANICAL    DRAFT, 


FIG.  30.     SPECIAL  STEEL-PLATE  STEAM  FAN  WITH  DOUBLE  ENCLOSED  ENGINE. 


MECHANICAL   DRAFT. 


259 


Still  another  form  of  the  steel-plate  steam  fan  with  a  double  enclosed  upright 
engine  is  shown  in  Fig.  30.  This  has  a  top  horizontal  discharge,  and  is  appli- 
cable for  either  forced  or  induced  draft.  The  engine  is  supported  upon  a  sub- 
stantial cast-iron  base  and  carries  the  fan  wheel  upon  its  extended  shaft.  The 
hand  wheel  upon  the  outer  end  of  the  shaft  is  provided  for  starting  the  engine 
off  the  centre,  when  necessary.  Large  numbers  of  fans  of  this  general  type, 
but  with  the  point  of  discharge  to  suit  the  conditions,  have  been  furnished  for 
the  production  of  draft. 

A  pair  of  down-discharge  fans  is  shown  in  Fig.  31,  the  combination  with  the 
engine  forming  a  duplex  steam  fan,  in  which  both  fans  are  oper- 

ated in  unison  by  the  same  engine.  gfc^JLrJL  |]  A  wheel  is  carried  on  each 


FIG. 


SPECIAL  DUPLEX  STEEL-PLATE    STEAM  FAN  WITH  DOUBLE  ENCLOSED   ENGINE. 


end  of  the  shaft,  which  is  provided  with  couplings  between  the  engine  bearings 
and  those  upon  the  fans,  so  that  the  engine  can  be  entirely  removed  without 
disturbing  the  fans.  By  the  arrangement  for  down  discharge  these  fans  may  be 
placed  above  the  boilers  and  the  air  delivered  directly  downward  to  them.  If  it 
be  a  stationary  plant,  a  duct  would  connect  each  outlet  to  the  boiler  ashpits,  but 
if  used  in  the  marine  service  either  the  closed  ashpit  or  closed  fire-room  system 
of  supply  could  be  adopted.  In  the  latter  case  the  air  would  simply  be  de- 
livered through  openings  in  the  deck  corresponding  to  the  outlets  of  the  fans 
and  thence  discharged  directly  downward  into  the  boiler  rooms.  The  duplex 
feature  reduces  the  height  which  it  would  be  necessary  to  provide  for  a  single 
fan  of  the  same  capacity. 


26o 


MECHANICAL   DRAFT. 


Double  Upright  Enclosed  Engine.  —  One  of  the  first  requisites  of  an  engine 
for  fan  propulsion  is  the  ability  to  operate  continuously  at  high  speed.  The  de- 
pendence which  is  placed  upon  the  fan  when  it  is  utilized  for  mechanical  draft 
is  such  that  perfection  in  the  engine  is  an  important  requisite.  For  moderate 
speeds  and  cleanly  surroundings  the  ^^  types  of  single  upright  engines  pre- 
viously described  effectually  serve  the  I  1  purpose.  But  where  the  speed  is 


continuous,  the  en- 
represented  in  Fig. 

ders  placed  side  by 
the  cranks    are  set 
ciprocating    parts 
high  speed  is  made 
are  of  large  diam- 
the  stroke,  so  that 
veloped  at  high  rel- 
speed.     The  steam 
ders   is   regulated 
valve,  and  au- 
prevent 
from 


excessive  and  the  operation 
gine  should  take  the  form 

32- 

This  type  has  two  cylin- 
side   in    the  same   casting, 
opposite  (at  180°),  the  re- 
are  thereby   balanced   and 
possible.     The    cylinders 
eter   as    compared    with 
great  power  may  be  de- 
ative  but  moderate  piston 
admission  to  both  cylin- 
by  a  single  balanced  pis- 
tomatic    relief    valves 
all    danger   of    damage 
water.     All  moving  parts 
subject  to  friction  are  of 
steel,  and  the  bearings 
are  of   ample  size. 
Complete 
sight- oiling 
arrange- 
ments from  a  sin- 
gle oil  tank  connect  with 
all  of  the  bearings.    The 
frame  is  so  made  as  to 
entirely  en- 
close all 
running 
parts,  still 
leaving  them 
accessible  by  merely 
opening  the  door. 


FIG.  32.    DOUBLE  UPRIGHT  ENCLOSED  ENGINE. 


MECHANICAL    DRAFT. 


261 


Special  Steel-Plate  Steam  Fan,  Double  Enclosed  Engine,  Cylinders  beneath 
the  Shaft.  —  Insufficient  height  above  the  centre  of  the  fan  shaft  frequently 
compels  the  substitution  of  an  engine  with  cylinders  beneath  the  shaft  for  one 

having  its  cylinders    above  the  shaft. 
The  same  general  type  of  engine  is 
employed,  however,  the    enclosed 
feature  being  retained,  and  the 
working  parts,  which  are  the 
same  as  those  in  the  other  type, 
being  made  readily  accessible. 
This  form  of   construction  is 
clearly  shown  in  Fig. 
33.     A  single 
oil   tank   sup- 
plies all  bear- 
ings  through 
connecting 
tubes  or  wip- 
ers.    The   en- 
top,  including  the 
door  and  oil  tank,  may 
be  easily  removed  if 
necessary.     The  auto- 
matic cylinder  relief 
valves,    which    are 
plainly  shown 
in  the  cut, 
form  an 
impor- 
I    tant  fea- 
ture of  this 

FIG.  33.     SPECIAL  STEEL-PLATE  STEAM  FAN  WITH  DOUBLE  ENCLOSED  ENGINE, 

CYLINDERS  BENEATH  THE  SHAFT.  ' 

which,  like 

those  previously  described,  is  of  steel  plate,  is  designed  to  discharge  directly 
downward  through  an  outlet  in  the  base  at  the  end  nearest  the  observer.  Such 
an  arrangement  is  particularly  convenient  for  application  under  many  of  the 
conditions  which  exists  in  mechanical  draft  plants,  where  the  air  is  to  be  forced 
into  the  ashpits. 


262 


MECHANICAL    DRAFT. 


FIG.  34.     SPECIAL  DUPLEX  STEEL-PLATE  STEAM  FAN  APPLIED  FOR  INDUCED  DRAFT. 


MECHANICAL   DRAFT. 


263 


Special  Duplex  Steel-Plate  Steam  Fan.  —  Two  fans  provided  with  engines  of 
the  type  just  described  and  set  up  in  proper  manner  to  form  a  duplex  fan  are 
shown  in  Fig.  34,  which  represents  the  arrangement  designed  for  and  installed 
at  the  Holyoke  Street  Railway  Co.'s  power  house  at  Holyoke,  Mass.,  for  the  pro- 
duction of  draft  by  the  induced  system.  The  flue  gases  enter  the  brick  chamber 

beneath  the  fans  and  thence  pass 
to  the  sheet-iron  connection 
between  them.     Here  is 
located,  as   shown,  a 
k    swinging  damper,  by 
•k   means  of  which  the 
HI  gases  may  be  made 
\  to  enter  either  fan. 
\  Another   damper, 
in  the  connection 
above   the  fans, 
likewise   operates 
so  that  when  one 
fan  is  in    use  the 
other    is    entirely 
shut  off   and  is  ac- 
cessible for  cleaning 
or    other   purposes. 
Either    fan    is 
capable  of   pro- 
ducing the  max- 
imum draft  that 
required   by 
the  entire  plant. 
One     fan     may 
thus    serve  as  a 
relay. 

Special  Steel-Plate  Steam  Fan  with  Upright  Compound  Engine.  —  The  fan 
illustrated  in  Fig.  35  differs  from  those  previously  shown,  in  that  it  is  provided 
with  a  special  type  of  compound  engine.  This  engine,  because  of  its  simplicity 
and  economical  performance,  is  of  value  where  the  exhaust  steam  cannot  be 
utilized  and  high  efficiency  is  important.  A  single  oscillating  valve  performs  the 
functions  of  the  two  valves  necessary  in  the  ordinary  types,  and  with  half  the 
complication  of  moving  parts. 


FIG.  35. 


SPECIAL  STEEL-PLATE  STEAM  FAN  WITH  UPRIGHT 
COMPOUND  ENGINE. 


264 


MECHANICAL    DRAFT. 


Special  Steel-Plate  Steam  Fan  with  Double  Open-Type  Engine.  —  Another 
type  of  double  upright  engine,  not  enclosed,  and  therefore  suitable  only  for 
cleanly  locations,  is  illustrated  in  Fig.  36.  The  relative  size  of  this  steam  fan 

can  be  judged  by  the  fact  that  the  engine  cyl- 
inders are   9  inches  in  di- 
ameter by   5^   inches 
stroke.      For    ab- 
solute rigidity, 
the  engine  was, 
in   the   plant 
from    which 
this  illustra- 
tion    was 
taken,    set 
upon  a  spe- 
cial   brick 

foundation. 

•• 

if   This    ren- 
JF  ders  its  sup- 
/  port   entirely 
independent 
)f  the  fan  and 
removes  all 
strain 
there- 
from. 
The 
wheel, 
as  in  pre- 
vious fans 
described,  is  over- 
hung   upon    the 
end  of  the  engine 

shaft,  which  is  of  large  diameter  and  supported  in  three  bearings  within  the 
base  of  the  engine.  The  greatest  care  has  been  given  to  the  continuous  and 
effectual  oiling  of  this  engine,  with  the  result  that  in  numerous  installations  on 
transatlantic  steamers  it  makes  the  passage  without  a  stop.  It  is  extremely 
compact,  requiring  the  minimum  of  floor  space  for  a  given  output,  and  is,  there- 
fore, especially  valuable  for  use  where  but  little  space  is  available. 


FIG.  36.     SPECIAL  STEEL-PLATE  STEAM  FAN  WITH  DOUBLE 
OPEN-TYPE  ENGINE. 


MECHANICAL   DRAFT. 


265 


Special  Cast-Iron  Steam  Fan  with  Double  Horizontal  Engine.  —  A  somewhat 
unique  form  is  presented  in  Fig.  37,  which  is  from  a  photograph  of  one  of  the 
fans  furnished  for  the  U.  S.  S.  Puritan.  The  side  pieces  of  the  shell  are  of 
cast  iron,  the  rim  being  of  heavy  steel  plate  and  the  entire  bottom  of  the  casing 

being  open  for  the  de-  ^^— -*••"-  _B livery  of   the   air  directly 

downward.     Sup-     ~^^M0^  ^^fe*.    ported  upon  projecting 

brackets  cast     *^Bl  „   &  ^  "O  ffl "ip  na  B~  \v      on   to  one   side  of 


the  shell 
zontal  en- 
cranks 


a   double    hori- 
gine    with   its 
set  opposite 
so  that  the 
recipro- 
cating 
parts 


FIG.  37.     SPECIAL  CAST-IRON  STEAM  FAN  WITH  DOUBLE  HORIZONTAL  ENGINE. 

are  balanced.  High  rotative  speed  is  thus  made  possible  without  objectionable 
vibration  of  the  engine.  The  engine,  in  fact,  consists  of  two  engines,  so  con- 
structed that  either  may  be  removed  without  disturbing  the  other.  The  valves, 
which  are  of  the  piston  type,  are  actuated  by  eccentrics,  transmitting  the  motion 
by  means  of  rockers.  A  thorough  system  of  sight-feed  oilers,  wipers  and  catch 
cups  is  provided,  as  is  clearly  shown. 


266 


MECHANICAL   DRAFT. 


MECHANICAL   DRAFT. 


267 


Steel-Plate  Steam  Fan  with  Three-Quarter  Housing.  —  All  the  fans  previously 
described  have  been  of  the  full-housing  type.     Under  certain  conditions,  such  as 
a  lack  of  available  height  or  the  desire  to  discharge  into  an  underground  duct, 
a  fan  having  a  portion  of  its  scroll  constructed  in  the  brick  foundation  is  both 
economical  and  convenient.     The  standard  type  of   three- 
quarter-housing  fan,  as  this  form  is  called,  is 
presented  in  Fig.  38,  where  it  is  shown 
as  driven  by  a  direct-connected 
horizontal  engine,  and  arranged 
^  to  deliver  the  air  into  an  un- 
•k  derground  duct.      Such  an 
arrangement  is  of  especial 
convenience   for    a   large 
forced-draft  plant,  where 
the  air  is  forced  into  the 
ashpits    from    a    duct 
H  beneath  or  in  front  of 
them.     For  an  induced- 
draft  plant  the 
arrangement 
"  shown  in  Fig.  39 
I  is  well  adapted. 
The    curve    of 
the  fan  scroll  is 
continued  with- 
in    the     brick 
foundation  and 
the  air   or  gas 
is  discharged 
horizontally  at 
the  top,  whence 
it  may  be  read- 
ily conducted 

to  a  chimney.  The  engine  is  of  the  single  upright  variety  already  illustrated  in 
connection  with  the  full-housing  fans.  It  carries  the  fan  wheel  upon  its  ex- 
tended shaft,  and  is  rigidly  supported  on  a  substantial  brick  foundation  bonded 
into  the  fan  foundation.  The  absence  of  a  bearing  in  the  inlet  leaves  it  entirely 
unobstructed  for  the  passage  of  air  or  gases,  the  condition  desirable  for  induced 
draft  adoption. 


FIG.  39.     STEEL-PLATE  STEAM  FAN  WITH  THREE-QUARTER  HOUSING 
AND  SINGLE  UPRIGHT  ENGINE. 


268 


MECHANICAL    DRAFT. 


Steel-Plate  Steam  Fan  with  Three-Quarter  Housing  and  Double  Upright  En- 
gine.—  One  of  the  most  important  features  desirable  in  a  steam  fan  applied  for 
induced  draft  is  an  engine  capable  of  sustained  operation  at  high  speed.  The 
high  speed  is  necessary  because  of  the  greater  tip  velocity  which  a  fan  must 
have  in  order  to  produce  a  given  pressure  or  vacuum  when  it  handles  hot  gases, 
while  the  necessity  of  ~~~~—° — r~~*-^.  continuous  operation  is  evident 

from  the  fact  that  an  ^^fKm     |aiii^,/vni      °x  ,  entire  establishment  may  be  de- 
pendent for  its 
produced.    Both 
vided  in  the 
Fig.  40.     The 
variety,  which 
trated  in  Fig. 
scribed.    The 


power  upon  the  draft  thus 
of  these  features  are  pro- 
type  of  fan  illustrated  in 
engine  is  of  the  double 
been  already  illus- 
32,  and  there  fully  de- 
large  diameter  of   its 
cylinders,  their  short 
stroke    and 
H^     the  cranks 
set  at  1 80° 
are  all  con- 
ducive to 
the  devel- 
opment 
of    the 
maximum 
power  for 
a  given 
space,  and 
the  main- 
tenance 
of  a  high 

relative  speed.  The  wide  spacing  of  the  journals  on  the  engine  insures  the 
stable  overhanging  of  the  wheel,  even  if  of  considerable  width.  Although  here 
shown  discharging  into  an  underground  duct,  a  fan  of  this  type  can  evidently  be 
as  readily  arranged  to  discharge  in  any  given  direction.  When  arranged  to  dis- 
charge the  air  or  gas  directly  upward,  it  becomes,  with  its  unobstructed  inlet, 
especially  adapted  for  induced  draft ;  for  the  outlet  may  be  prolonged  into  a 
short  stack  which  will  be  supported  by  the  fan.  Any  of  these  fans  can  be  fitted 
with  pulleys  instead  of  engines,  and  arranged  to  be  driven  by  belt  from  any  de- 
sirable source  of  power. 


FIG.  40.     STEEL-PLATE  STEAM  FAN  WITH  THREE-QUARTER  HOUSING  AND 
DOUBLE  UPRIGHT  ENGINE. 


MECHANICAL   DRAFT. 


269 


Steel-Plate  Steam  Fan,  Three-Quarter-Housing  Type,  with  Steel-Plate  Bottom. 

—  The  condition  frequently  presents  itself  in  induced  draft  practice  where  the 
fan  is  to  be  set  above  the  ground  floor,  and  yet  it  is  desired  that  it  be  of  the 
three-quarter-housing  type.  It  then  becomes  necessary  to  construct  the  bottom 
of  steel  plate  in  place  of  masonry.  Such  an  arrangement  is  shown  in  Fig.  41. 
As  shown,  the  I  beams  of  the  regular  floor  con- 
struction are  so  disposed  as  to  furnish 
the  proper  support  for  the  fan  and 
its  engine,  while  permitting  of  the 
downward  projection  of  the  lower 
part  of  the  housing.  By  a  variety 
in  the  heights  of  the  beds  for  dif- 
ferent-sized engines  it  is  possible 
to  locate  the  centre  of  such 
a  fan  at  almost  any  desired 
distance  above  the  floor.  A 
complete  duplex  induced 
draft  plant,  with  fans  of 
this  general  type,  is 
presented  in  Fig.  42. 
Here  the  engines  are 
horizontal  and  the 
shafts  extend  through 
the  fan  wheels  to  a 
central  chamber  which 
is  kept  open  to  the  at- 
mosphere and  where 

FIG.  41.     STEEL-PLATE  STEAM  FAN,  THREE-QUARTER-HOUSING       6^ 

TYPE,  WITH  STEEL-PLATE  BOTTOM.  water-cooled  boxes. 

The  fan  outlets  are  an- 
gular so  as  to  permit  of  more  ready  connection  to  the  stack  above.  The  space 
between  the  fans  and  towards  the  observer  forms  a  chamber  to  which  the  gases 
are  conducted  from  the  boiler.  Here  the  adjustable  damper  serves  to  admit 
them  to  either  fan  at  will,  while  the  dampers  in  the  connections  above  operate 
to  open  or  close  the  corresponding  outlets.  As  each  fan  is  capable  of  producing 
all  the  draft  required  for  the  entire  plant,  either  may  be  stopped  when  desired, 
the  dampers  so  adjusted  that  no  gases  will  pass  through,  and  the  interior  thus 
rendered  accessible.  The  stack  is  supported  on  the  special  steel-beam  con- 
struction which  is  practically  independent  of  the  fans. 


270 


MECHANICAL    DRAFT. 


FIG.  42.     SPECIAL  DUPLEX  STEEL-PLATE  STEAM  FAN,  THREE-QUARTER-HOUSING  TYPE, 
WITH  STEEL-PLATE  BOTTOM  AND  HORIZONTAL  ENGINES. 


MECHANICAL    DRAFT. 


271 


Electric  Fan,  "Monogram"  Pattern.  —  Conditions  may  arise  in  boiler  practice 
in  which  an  electric  fan  will  prove  of  especial  value  for  draft  production.  Such 
a  condition  frequently  exists  where  the  boilers  are  devoted  solely  to  producing 
steam  of  low  pressure  for  heating  purposes,  but  where  the  draft  is  insufficient. 
Manifestly  the  low  pressure  makes  it  practically  impossible  to  introduce  a  steam 
fan  to  assist  in  the  draft  production,  unless  the  engine  have  a  cylinder  of  extraor- 
dinary size.  An  electric  fan,  however,  if  installed  where  a  power  circuit  is 
constantly  available,  becomes  prac-  ^^fjf^^^^  tically  independent  of  the 
boiler  itself  and  may  be  as  read-  .^gl  Ife^  *ty  °Perated  when 

starting   the    fires    as   when^^H  jJWi  i        9^     steam  is  up.     In 

small  plants,  with  possibly    Sj|H  SPJ^A       only    a    single 


boiler,  the  purchase  of   a 
even  if    the  steam    pres- 
sufficient,  is  almost  out  of 
tion ;   for  if  made   small 
do    only    the    work    re- 
it  becomes    almost    im- 
But    an    elec- 
be  furnished 


steam  fan, 
sure     is 
the  ques- 
enough  to 
quired  of  it, 
practicable, 
trie  fan  may 
at  a  reason- 

;i!,lc     <:os!         J^fe  IL  ^SjljjfS         and  of   ac- 

ceptable ef-      jH't-fV'  I  ficiency,  in 

compara-         •!%-  tivelY  sma11 

size.      Such       tlf/!^:  a  fan  is  that 

illustrated         "'^p ',  ..,,-,;  4fe  -  *|:,     in  Fig.  43. 

The  shell  is  ' ;< -^ ^HjlB^^B*^^'^  °f  cast  iron, 

and  of  the  ^:%  Monogram 

type  already  illustrated 

in   Fig.    1 6.  FIG.  43.     ELECTRIC  FAN,  "MONOGRAM"  PATTERN.  It  furnishes 

a  very  rigid  structure,  to 

which  the  motor  is  attached.  This  is  of  the  bi-polar  type,  substantially  contained 
within  the  heavy  wrought-iron  circular  fieldpiece.  The  shaft  runs  in  two  ring- 
oiler  bearings,  and  throughout  the  machine  is  the  product  of  the  most  careful 
design  and  construction.  Its  form  permits  of  the  fan  being  placed  in  any  posi- 
tion desired,  with  the  feet  uppermost,  for  instance,  or  attached  to  a  vertical  wall, 
so  that  the  air  may  be  discharged  directly  upward  or  downward,  according  as 
the  fan  may  be  set.  The  application  of  such  a  fan  may  save  almost  the  entire 
cost  of  an  additional  boiler  when  a  sufficiently  high  rate  of  combustion  to 
accomplish  the  desired  results  in  the  generation  of  steam  is  impossible  under 
existing  conditions. 


272 


MECHANICAL    DRAFT. 


Electric  Fan,  Steel-Plate  Pattern.  —  For  comparatively  large  plants  where  an 
electric  fan  is  employed  the  pattern  presented  in  Fig.  44  is  well  adapted.  The 
shell  is  of  the  same  steel-plate  construction  as  the  steel-plate  exhauster  already 


FIG.  44.    ELECTRIC  FAN,  STEEL-PLATE  PATTERN. 

shown  in  Fig.  22.  The  motor  shown  is  of  the  independent  bi-polar  type,  but 
in  large  sizes  a  multi-polar  machine  is  employed.  For  small  sizes  the  circular 
form  shown  in  the  preceding  illustration  is  also  adaptable.  This  type  lends 
itself  to  a  great  variety  of  arrangements. 


CHAPTER    XIII. 

APPLICATION   OF   THE   STURTEVANT  FANS   FOR    MECHANICAL 

DRAFT. 

The  principles  of  mechanical  draft  have  already  been  discussed,  its  advan- 
tages shown  and  various  types  of  Sturtevant  fans  designed  for  this  specific 
work  have  been  presented.  It  now  remains  to  illustrate  the  manner  of  applica- 
tion of  such  fans,  and  to  indicate  the  economical  results  obtained  by  their  use. 
It  has  already  been  stated  that  this  Company  advocates  no  particular  system 
or  method  of  application  to  the  exclusion  of  others,  but  that  it  is  interested  in 
all  arrangements  of  which  a  fan  forms  a  part. 

In  the  most  impartial  manner  possible  there  are  here  presented  descriptions 
and  reports  of  tests  of  various  systems,  together  with  illustrations  of  the  plants 
themselves  and  details  of  the  application.  Where  a  fan  alone  is  concerned  this 
Company  is  alone  responsible  (provided  that  the  fan  is  properly  operated),  and 
assumes  full  credit  for  the  results.  But  where  the  fan,  of  Sturtevant  manufac- 
ture, is  used  in  connection  with  air  or  water  heaters,  stokers,  special  grates,  or 
the  like,  introduced  by  others  as  a  part  of  their  system,  responsibility  and  credit 
are  assumed  only  in  so  far  as  they  directly  relate  to  the  fan,  although  the  system 
may  in  reality  be  entirely  dependent  upon  the  use  of  the  fan  for  its  successful 
operation.  Therefore,  the  statements  regarding  such  systems  are,  so  far  as 
possible,  quoted  from  reliable  sources  and  the  references  given.  This  is  done 
in  the  earnest  endeavor  to  present  each  independently  upon  its  merits,  and 
thereby  avoid  all  appearance  of  a  tendency  to  draw  comparisons. 

In  the  selection  of  these  various  plants  for  presentation  the  object  has  been, 
first,  to  present  only  those  of  which  the  Sturtevant  fan  forms  a  part,  and, 
second,  to  make  them  illustrative  of  as  many  different  arrangements  as  possible. 
It  has  been  extremely  difficult  to  choose  between  the  large  number  of  plants 
which  have  been  installed,  and  therefore  those  presented  must  be  considered  as 
only  typical  of  the  classes  which  they  represent.  They  all,  however,  serve  to 
show  in  their  various  ways  the  manifest  advantages  of  mechanical  draft,  and  to 
at  least  suggest  the  arrangements  which  are  possible  under  the  different  condi- 
tions which  present  themselves  in  practice.  They  are  likewise  evidence  of  the 
ability  of  this  Company  to  furnish  fans  and  engines  in  many  types  and  sizes, 
each  best  adapted  to  the  particular  requirements. 


274 


MECHANICAL    DRAFT. 


Z  Q 

<  w 


MECHANICAL    DRAFT. 


275 


Typical  Arrangement  of  the  Sturtevant  Steam  Fan  for  the  Production  of 
Under-Grate  Forced  Draft. —  In  Fig.  45  is  presented  one  of  the  simplest  arrange- 
ments of  a  mechanical  draft  plant.  The  fan,  which  is  of  the  steel-plate  pattern 
with  angular  downward  discharge,  is  driven  by  a  direct-connected  enclosed 
double  upright  engine.  The  air  is  discharged  through  the  entire  open  bottom 
into  the  underground  brick  duct  extending  along  the  front  of  the  battery  of 
boilers.  From  this  duct  smaller  branches,  two  to  each  boiler,  extend  to  the 
ashpits,  to  which  the  air  is  admitted  in  the  requisite  amount  through  ashpit 
dampers  of  the  type  shown  in  Fig.  46.  There  is  thus  maintained  within  the 
ducts  and  ashpits  a  pressure  greater  than  that  of  the  atmosphere  by  an  amount 
dependent  upon  the  speed  of  the  fan,  which  may  be  regulated  at  will.  If  con- 
venient in  such  an  arrangement,  the  main  duct  can  be  carried  immediately  be- 
neath the  boilers  or  at  any  desirable  distance  in  front  of  them.  The  fan  may 
be  as  readily  placed  in  any  other  position  than  that  shown,  and,  in  case  floor 
space  is  not  available,  may  be  located  on  top  of  the  boilers  and  arranged  to  dis- 
charge directly  downward  into  the  underground  duct.  In  fact,  the  possible 
arrangements  are  almost  without  number,  for  the  fan  may  be  of  any  type,  driven 
by  direct-connected  engine  or  electric  motor,  or  by  belt  from  an  independent 
engine,  or  from  a  line  shaft,  and  may  be  located  in  any  convenient  position. 

Ashpit  Dampers.  —  The  success  of  an  under-grate  system  of  forced  draft  de- 
pends largely  upon  the  method  adopted  for  admitting  the  air  to  and  distributing 
it  within  the  ashpit.  Under  ordinary  conditions  it  will  not  do  to  admit  it 
through  the  bottom  and  allow  it  to 
be  forced  directly  upward 
against  the  grates.  It  must 
first  be  so  deflected  as  to 
move  upward  in  substan- 
tially equal  vol- 
ume at  all  parts 
of  the  grate. 
To  accomplish 
this  result  the 
form  of  damper 
shown  in  Fig. 
46  is  employed. 
As  here  shown, 
it  is  placed  in 
the  sloping  front 
of  the  ashpit  bot-  FIG.  46.  STURTKVANT  ASHPIT  DAMPER  IN  BOTTOM  OF  ASHPIT. 


276 


MECHANICAL    DRAFT. 


torn  and  consists  of  a  frame  thoroughly  imbedded  in  the  brickwork,  a  door 
hinged  thereto  and  a  handle  bar  for  operating  it.  The  door  is  countersunk  in 
the  frame  so  that  when  closed  the  top  is  perfectly  flush  and  the  ashes  may  be 
readily  raked  over  it.  The  handle  bar  is  notched  so  as  to  permit  of  adjustment 
to  suit  the  conditions.  In  the  form  here  shown  the  arrangement  is  readily  ap- 
plicable to  an  old  boiler,  for  the  ducts  may  be  easily  constructed  without 
affecting  the  boiler  setting,  and  the  handle  bar  may  be  introduced  through  a  sin- 
gle opening  which  may  be  readily  made  in  the  boiler  front. 

When  the  mechanical  draft  plant  forms  a  part  of  the  original  installation,  that 
is,  when  the  use  of  mechanical  draft  is  decided  upon  before  the  boilers  are  erected, 
the  type  and  arrangement  of  ashpit  damper  shown  in  Fig.  47  is  exceedingly 
convenient.  The  air  duct  is  in  this  case  constructed  within  the  bridge  wall,  and 
the  dampers  (one  or  more  to  each  boiler)  are  placed  as  shown.  Each  damper 
is  provided  with  an  extended  shaft  and  lever  so  that  it  may  be  operated  from  the 
boiler  front  by  means  of  the  long  handle  bar.  This  is  placed  at  the  side,  so  as 
not  to  interfere  with  the  doors.  As  the  air  leaves  the  duct  and  passes  through 
the  damper 
it  is  turned 
downward. 
It  spreads 
over  the  en- 
tire bottom 
of  the  ash- 
pit, thence 
rising  in  an 
even  vol- 
ume at  low 
velocity  to 
the  grate 

above.  All  possibility  of  local  air  blast  upon  the  grate  is  thus  avoided,  and 
the  combustion  is  greatly  accelerated,  but  maintained  perfectly  regular. 

The  importance  of  proper  arrangements  for  admission  and  distribution  of  the 
air  cannot  be  too  greatly  emphasized,  for  upon  them  practically  depends  the 
success  of  the  system.  By  the  introduction  of  reasonably  large  ducts  and 
branches  the  velocity  can  be  kept  sufficiently  low  to  prevent  its  direct  impinge- 
ment upon  any  portion  of  the  grate  when  it  escapes  from  the  provided  opening 
into  the  ashpit.  The  adjustable  feature  of  the  damper,  together  with  any  device 
which  may  be  provided  for  proportioning  the  speed  of  the  fan  to  the  draft  require- 
ments, makes  it  possible  to  absolutely  control  the  air  supply  and  draft  pressure. 


FIG.  47.     STURTEVANT  ASHPIT  DAMPER  IN  BRIDGE  WALL. 


MECHANICAL    DRAFT. 


277 


Crystal  Water  Company,  Buffalo,  N.  Y. —  The  arrangement  illustrated  in  Fig. 
48  serves  to  emphasize  the  simplicity  of  a  forced-draft  application  to  a  small 
boiler  plant,  and  is  an  excellent  illustration  of  an  under-grate  system.  It  con- 
sists of  two  100  horse-power  horizontal  return  tubular  boilers  supplied  by  a 
regular  Sturtevant  3  x  4^  steel-plate  steam  fan.  The  air  from  the  fan  is  dis- 
charged into  a  duct  extending  immediately  behind  and  formed  in  part  by  the 
bridge  walls  of  both  boilers,  while  the  air  is  admitted  to  the  ashpits  through 


FIG.  48.     FORCED-DRAFT  PLANT  AT  CRYSTAL  WATER  COMPANY,  BUFFALO,  N.  Y. 

special  dampers  operated  by  levers,  substantially  as  shown  in  Fig.  47.  One 
of  the  levers,  extending  forward  to  the  front  of  the  boiler,  is  shown  in  the  cut. 
The  speed  of  the  engine  is  controlled  by  a  series  of  regulating  and  reducing 
valves  introduced  under  the  Beckman  system. 

It  is  stated1  that  "by  this  system  we  are  able  to  hold  our  steam  pressure 


Crystal  Water  Company,  Buffalo,  X.  Y.     Letter  of  March  12,  1896,  to  B.  F.  Sturtevant  Co. 


278 


MECHANICAL    DRAFT. 


MECHANICAL    DRAFT.  279 

within  four  or  five  pounds  of  a  given  point  and  a  large  saving  in  steam  is 
effected,  as  the  blower  runs  at  low  speed  most  of  the  time.  We  use  a  four-to- 
one  mixture  of  hard-coal  screenings  and  soft  run  of  mine,  costing  us  about  30 
cents  less  per  ton  than  clear  soft  coal,  while  entirely  obviating  the  smoke  cus- 
tomary where  soft  coal  is  used  clear.  Our  boiler  capacity  is  largely  increased, 
and  our  labor  of  firing  is  considerably  lessened. 

"Our  draft  pressure  is  from  ^  to  i}^  inches  of  water,  according  to  the  re- 
quirements. We  consume  from  20  to  30  pounds  of  coal  per  square  foot  of  grate 
surface  per  hour  when  running  under  full  load  at  80  pounds  steam  pressure  and 
a  speed  of  from  400  to  480  revolutions  per  minute.  We  have  a  large  stack 
with  good  natural  draft,  but  can  neither  burn  the  coal  as  efficiently  nor  as  fast 
as  necessary  when  forced  draft  is  not  in  use." 

As  indicative  of  the  efficiency  of  mechanical  draft  to  prevent  smoke,  the  fol- 
lowing statement1  is  presented :  "  The  city  has  recently  passed  a  smoke  ordi- 
nance compelling  all  concerns  to  put  on  smoke  consumers.  The  committee, 
after  examining  our  plant  a  few  days  since,  decided  that  it  was  not  necessary 
for  us  to  put  on  a  smoke  consumer  on  account  of  the  forced  draft  used." 

B.  F.  Sturtevant  Co.,  Jamaica  Plain,  Mass. —  This  plant  presents  an  excellent 
exemplification  of  a  number  of  the  advantages  of  mechanical  draft.  Radical 
changes  in  the  arrangement  of  the  works  necessitated  the  removal  of  the  boiler 
plant  to  a  position  over  100  feet  from  its  original  location  and  that  of  the 
chimney  which  had  previously  served  to  produce  the  draft.  The  distance  was 
too  great  to  permit  of  the  further  use  of  the  chimney,  and  its  removal  was  out 
of  the  question.  It  was  therefore  permitted  to  stand  as  a  useless  monument  to 
a  discarded  method  of  draft  production.  A  new  chimney  in  the  desired  loca- 
tion would  have  cost  about  $1,450,  and  would  have  occupied  valuable  floor 
area.  It  was,  therefore,  decided  to  introduce  a  fan  driven  by  direct-connected 
engine  and  designed  to  operate  on  the  induced  system.  Its  market  value,  com- 
plete with  the  stack,  was  about  $700,  which  may  be  compared  with  that  of  the 
chimney,  manifestly  to  the  disadvantage  of  the  latter.  The  fan  was  placed 
above  the  boilers  and  discharged  through  a  small  short  stack  extending  just 
above  the  roof.  It  was  provided  with  a  by-pass  for  emergencies.  The  relative 
proportions  of  the  stack  and  chimney  are  clearly  shown  in  Fig.  49,  which  pre- 
sents a  perspective  view  of  the  front  side  of  the  works.  The  stack  appears 
just  beyond  and  to  the  right  of  the  chimney.  Comment  is  unnecessary. 

The  boiler  plant,  which  is  of  about  260  horse-power  nominal  rating,  consists 
of  three  boilers  arranged  as  shown  in  Fig.  50.  Two  of  these  boilers  are  66 


Crystal  Water  Company,  Buffalo,  N.  Y.     Letter  of  July  30,  1897,  to  B.  F.  Sturtevant  Co. 


28o 


MECHANICAL    DRAFT. 


FIG.  50.      INDUCED-DRAFT  PLANT  AT  B.  F.  STURTEVANT  Co.'s,  JAMAICA  PLAIN,  MASS. 


MECHANICAL    DRAFT. 


281 


inches  in  diameter,  each  containing  123  tubes  3  inches  in  diameter  by  15  feet 
long.  The  third  boiler  is  60  inches  in  diameter,  and  contains  64  tubes  3^ 
inches  in  diameter  by  15  feet  long. 

The  fan  is  a  special  go-inch  Sturtevant  exhauster,  to  which  is  attached,  upon 
the  farther  side,  and  not  visible  in  the  illustration,  a  5  x  4  double  upright  Stur- 
tevant engine.  The  speed  of  the  engine,  and  consequently  the  intensity  of  the 
draft,  is  regulated  by  a  special  device  which  serves  to  open  the  throttle  and 
increase  the  steam  supply  in  proportion  as  the  steam  pressure  is  reduced.  It  is 
so  adjusted  that  a  decrease  of  about  one  pound  in  the  steam  pressure  serves  to 
change  the  speed  from  minimum  to  maximum,  or  nice  versa.  The  correspond- 
ing steam  and  draft  pressures  under  working  conditions,  as  shown  by  continuous 
graphic  records  from  special  instruments,  are  presented  later  in  Figs.  51  and  52. 

Shortly  after  the  installation  of  this  plant,  in  which  the  surface  ratio  was  not 
changed  from  that  previously  existing  with  chimney  draft,  comparative  tests 
were  made  under  the  direction  of  Prof.  Peter  Schwamb,  of  the  Massachusetts 
Institute  of  Technology. 

Three  separate  tests  were  conducted  upon  successive  days,  under  the  follow- 
ing conditions  :  — 

Test  No.  i.  Run  on  middle  boiler,  dampers  to  other  two  boilers  closed  and 
packed,  fires  drawn,  and  all  steam  and  water  connections  broken.  Fan  engine 
under  control  of  special  regulator.  Coal  as  indicated  in  Table  No.  121. 

Test  No.  2.  Conditions  same  as  in  No.  i,  except  that  fan  engine  was  fitted 
with  Water's  governor,  and  damper  in  inlet  connection  to  fan  was  operated  by  a 
Locke  damper  regulator.  Coal  as  indicated  in  Table  No.  121. 

Test  No.  3.  Run  on  three  boilers.  Fan  engine  regulated  as  in  No.  i.  Fire 
even,  not  drawn,  but  running  start  and  stop  were  made.  Coal  as  indicated  in 
Table  No.  121. 

The  air  supply  was  determined  by  anemometer  readings  at  ashpit  doors. 

Table  No.   121.  — Analysis  of  Coal,  Coal  and  Ash  (on  Grate)  and  Ash  — B.  F. 
Sturtevant  Co.  Test. 


Coal  and    1 

Number 

COAL  ANALYSIS. 

Ash  on  Grate 
at  Close  of 

Ash. 

• 

c 

H 

Moisture. 

or  Earthy               O 
Matter. 

N 

C 

C 

I 

81.9 

5-18 

0.5 

7.6l 

3-3 

i-5 

21.8 

30-52 

2 

81.6 

5.0 

1.04 

7-47 

3-2 

1-7 

21.3 

35.65 

3 

76-3 

2.0 

5-4 

12.3 









282 


MECHANICAL    DRAFT 


The  coal  used  in  Tests  Nos.  i  and  2  was  clear  George's  Creek  Cumberland, 
costing  $3.65  per  ton  of  2,000  pounds.  In  Test  No.  3  this  was  mixed  half-and- 
half  with  yard  screenings  costing  $2.00  per  ton,  making  the  cost  of  the  mixture 
$2.82^  per  ton. 

Table   122.  — Results  of  Tests  of  Induced-Draft  Plant  at  B.  F.  Sturtevant  Co., 
Jamaica  Plain,  Mass. 


Test  No.  i. 

Test  No.  2.    Test  No.  3. 

Duration          .         .  '        .         .         .         .         .         .           hours, 

11.5 

11.52 

11.55 

Average  steam-gauge  pressure         ....        pounds, 

79-4 

79-3 

78.9 

Average  temperature  of  feed  water         .         .         .        degrees, 

175-3 

189.5 

148.6 

Grate  surface           square  feet, 

30-25 

30.25 

85-5 

Water-heating  surface     square  feet, 

1.473-4 

1,473-4 

3,884.6 

Nominal  rating,  at  15  square  feet  of   heating  j  , 
surface  per  horse-power,                                   \ 

98.2 

98.2 

258.9 

Ratio  of  water-heating  surface  to  grate  surface 

48.7 

48.7 

45-4 

Coal  burned,  including  coal  equivalent  of  wood     .         pounds, 

7,461.7 

5.723.3 

r3,6i3-5 

Total  combustible  consumed           ....         pounds, 

7,005.8 

5.263.3 

12,691.1 

Air  per  pound  of  coal  burned         ....         pounds, 

20.12 

20.80 

21.81 

Air  per  pound  of  combustible         ....         pounds, 

21.43 

22.62 

23-39 

Quality  of  steam,  saturated  steam  taken  as  unity  .         . 

0.996 

0.995 

o-995 

Water  actually  evaporated,  corrected  for  quality  )                      , 
of  steam,                                                           \           pou 

70,1174 

54,862. 

114,568. 

^tromLdat6;!6"130^611   ^   ^              \          Pounds'   75,3*6-9 

58,115- 

126,313. 

Equivalent  water  evaporated  into  dry  steam  from  \                     , 
and  at  2120,  per  pound  of  coal  burned,         \ 

IO.IO 

10.15 

9.28 

Equivalent  water  evaporated  into  dry  steam  from  )                     , 
and  at  2120  per  pound  of  combustible,         J 

10.75 

11.04 

9-95 

Actual  horse-power  at  34.5  pounds  per  hour,  \  , 
from  and  at  2  1  20  per  horse-power,                          rse-power, 

189.6 

146.3 

3  '7-o 

Coal  burned  per  square  foot  of  grate  per  hour       .         pounds, 

21.45 

16.45 

15.09 

Water  evaporated  from  and  at  2120  per  square  )                     , 
foot  of  heating  surface  per  hour,                    }           pounds, 

4-45 

3-43 

3.08 

Average  draft  pressure  in  ashpit    .         .         .  inches  of  water, 

0.144 

0.078 

0.057 

Average  draft  pressure  at  back       .         .         .  inches  of  water> 

0.640 

0.372 

0.581 

Average  draft  pressure  at  fan          .         .         .  inches  of  water, 

1-253 

1-743 

1.342 

Temperature  of  boiler  room  .         .         ....        degrees, 

80.9            77.7 

80.4 

Temperature  at  uptake  .         .         ,   ,      „                 .        degrees, 

473-9           436-3 

468.4 

Temperature  at  fan          .         .         .         *        •.         .        degrees, 

389-5 

332-5 

438.7 

Revolutions  of  fan  per  minute        .         .'-     ^ 

2QI  6 

AO7 

-3  C2  7 

Steam  consumed  by  fan  engine  per  hour        .         .         pounds, 

£,\j  l.U 

272.3 

qpjj 

425-3 

323 

438.2 

Steam  used  by  fan  engine  per  pound  of  water  )                     , 
evaporated  (dry  steam,  212°)                            ( 

0.0416 

0.0769 

0.0366 

MECHANICAL   DRAFT. 


283 


The  general  conditions  and  results  of  the  tests  are  given  in  Table  No.  122. 
The  purpose  of  producing  all  the  steam  by  means  of  one  boiler  in  Tests  Nos.  i 
and  2  was  to  give  an  opportunity  to  force  the  boiler  well  above  its  rated  capac- 
ity. As  a  result,  in  Test  No.  i  it  exceeded  its  rating  by  93  per  cent,  and  in 
Test  No.  2  by  49  per  cent.  These  values  are  to  be  taken  into  account  in  con- 
sidering the  economic  results.  The  coal  burned  per  square  foot  of  grate  was  in 
the  first  test  brought  up  to  21.45  pounds;  but,  owing  to  the  fact  that  no  special 
arrangement  of  the  grates  was  adopted,  the  air  supply  was  20.12  pounds  per 
pound  of  coal,  somewhat  higher  than  it  might  otherwise  have  been.  The  evap- 
oration from  and  at  212°  per  pound  of  combustible  shows  a  difference  of  only 
about  2^  per  cent  between  the  two  tests,  even  under  the  different  conditions. 

The  difference  in  the  steam  required  to  operate  the  fan  engine  under  the 
three  tests  is  to.be  noted.  In  Test  No.  i  the  engine  was  automatically  con- 
trolled in  the  usual  manner  to  suit  the  requirements,  but  in  Test  No.  2  the  fan, 
operating  at  constant  speed  under  the  influence  of  the  Water's  governor,  pre- 
sented approximately  the  condition  with  a  chimney  which  produces  a  constant 
maximum  intensity  of  draft  which  is  adapted  to  the  requirements  by  shutting 
off  a  portion  of  the  volume  by  means  of  a  damper.  As  a  consequence,  the 
steam  expenditure  for  draft  is  greater  in  the  latter  case.  The  decreased  relative 
amount  of  steam  used  to  operate  the  fan  with  a  larger  plant  is  shown  by  Test 
No.  3.  As  in  the  original  design  it  was  determined  that  the  exhausf  steam 
from  the  fan  engine  was  to  be  utilized  for  heating  the  feed-water,  the  question 
of  efficiency  of  the  engine  was -of  minor  account.  In  fact,  with  this  in  view  an 
engine  of  simple  construction,  capable  of  continuous  operation,  was  adopted, 
rather  than  one  which  would  show  higher  efficiency  but  not  be  so  serviceable  for 
this  work.  Hence  the  somewhat  high  values  for  the  water  per  horse-power  per 
hour,  shown  in  Table  No.  123,  are  not  surprising.  Had  the  consumption  been 


Table  No.    123.  —  Results  of  Tests  of  Fan  Engine  at 
Jamaica  Plain,  Mass. 


F.  Sturtevant  Co.'s, 


DESIGNATION  OF  TEST. 

A 

p, 

C 

D 

Duration     ..... 

minutes,  seconds, 

11:0 

2:15 

10:40 

8:55 

Revolutions  per  minute 

438 

449 

443 

436 

Temperature  at  fan    . 

degrees, 

394 

387 

392 

366 

Indicated  horse-power 

8.20 

8.26 

7-99 

8.22 

Water  per  I.  H.  P.  per  hour 

pounds, 

63-5 

65-4 

66.9 

64.4 

284 


MECHANICAL    DRAFT. 


FIG.  51.     STEAM-PRESSURE  CHART  FROM  BOILER  PLANT  AT  B.  F.  STURTEVANT  Co.'s, 
JAMAICA  PLAIN,  MASS. 


MECHANICAL    DRAFT. 


285 


FIG.  52.     DRAFT-PRESSURE  CHART  FROM  INDUCED-DRAFT  PLANT  AT  B.  F.  STURTEVANT 
Co.'s,  JAMAICA  PLAIN,  MASS. 


286 


MECHANICAL    DRAFT. 


only  30  pounds,  the  fan  engine  would  have  required  in  Test  No.  3  only  1.7  per 
cent  of  the  total  steam  produced  by  the  boiler  plant.  The  results  of  tests  of 
other  and  larger  plants,  presented  in  this  chapter,  show  much  more  economic 
relations. 

The  efficiency  of  the  combustion  is  manifestly  one  of  the  most  important 
matters  in  connection  with  a  boiler  plant.  In  the  case  of  this  test,  flue-gas 
samples  were  continuously  taken  from  a  point  at  the  back  end  of  the  boilers 
where  the  gases  entered  the  tubes,  and  also  in  the  uptake  just  after  they  had 
left  the  tubes.  These  samples  were  analyzed  by  means  of  the  Orsat  apparatus, 
and  the  results  obtained  as  presented  in  Table  124.  The  amount  of  carbonic 
oxide  (CO)  was  at  all  times  too  small  to  be  detected  by  the  apparatus.  Numer- 
ous observations  of  the  top  of  the  chimney  stack  also  failed  to  show  the  pres- 
ence of  smoke,  except  for  a  few  seconds  after  each  firing.  The  photograph 
from  which  Fig.  49  was  reproduced  was  taken  during  working  hours,  and  serves 
to  show  the  absence  of  smoke. 

Table  No.  124.  — Analysis  of  Continuous  Gas  Samples  — B.  F.  Sturtevant  Co.  Test. 


Number 

Duration. 

BACK  END. 

UPTAKE. 

Test. 

Hours. 

CO2 

0 

CO 

N  + 

C02 

O 

CO 

N  + 

, 

9 

6.72 

10.72 

o 

81-33 

5.81 

13-53 

0 

80.68 

2 

10 

7-50 

11.09 

o 

81.31 

5.80 

13.88 

0 

80.32 

3 

9-5 

6.7I 

12.78 

0 

81-57 

5-75 

14.14 

o 

80.  1  1 

In  connection  with  this  plant  there  is  in  operation  a  continuous  steam  pres- 
sure recorder  and  a  mechanical  draft  recorder  of  the  form  previously  illustrated 
in  Fig.  9.  This  latter  is  connected  to  the  inlet  pipe  which  leads  to  the  fan,  and, 
therefore,  records  the  total  pressure  rather  than  the  difference  between  over 
and  under-grate  pressures.  Two  typical  and  corresponding  charts  from  these  in- 
struments are  reproduced  in  Figs.  51  and  52.  The  steam  pressure  is  maintained 
almost  absolutely  constant,  while  the  draft  pressure  necessary  to  secure  this 
result  varies  suddenly  and  between  considerable  limits.  The  instant  response 
of  the  fan  and  the  range  through  which  it  operates  are  evidently  essential  to 
the  results  obtained.  The  fluctuations  of  the  fan  speed  coincide  closely  with 
the  periods  of  firing,  and  thus  serve  to  furnish  a  large  volume  of  air  under 
greater  intensity  at  just  the  instant  when  most  required  because  of  the  increased 
discharge  of  inflammable  gases. 


MECHANICAL    DRAFT. 


287 


Table  No.  125.  —  Results  of  Full-Power  Forced-Draft  Contract  Trials  of  Vessels  of 
U.  S.  Navy  Equipped  with  Sturtevant  Fans. 


CLASS  AND 
Dimensions. 
NAME. 

Displace- 
ment, 

Mean 
I.  H.  P.  of 
all  Ma- 
chinery. 

Speed 
per  Hour, 
in  Knots. 

Number 
of  Stur- 
tevant 
Blowers 
on  Vessel. 

Total 
H.  P.  Re- 
quired to 
Drive 
Blowers. 

Air 
Pressure 
in  Fire 
Room,  in 
Inches 
of  Water. 

Coal 
per  Hour 
per 
Square 
Foot  of 
Grate,  in 
Pounds. 

L.  257  ft.    o  in. 

Cruiser  Detroit,      B.    37ft.  oin. 

2,068 

5.227 

18.71 

6 

18.9 

0.8 

D.    14  ft.    5Min. 

Cruiser 
Montgomery, 

L   257  ft.    o  in. 
B.    37ft.    oin. 
D.    14  ft.    oKin. 

2,091 

5.527 

19.06 

6 

1.4 

Gunboat 
Machias, 

L.  190  ft.    oin. 
B.    32  ft.    oin. 
D.    1  2  ft.    o%in. 

1,  068          1,873 

15-46 

2 

23.2           0.47            38.08 

Gunboat 
Castine, 

L.  190  ft.    o  in. 
B.    32ft.    oin. 
D.    .2ft.    oJiin. 

1,  068 

2,199 

16.03 

2 

16.9 

42.6 

Monitor 
Monadnock, 

L.  259  ft.    6  in. 
B.    55ft.  10  in. 

3.990 

3,000 

14-5 

2 

L).    1411.    6m. 

Cruiser 
New  York, 

L.  380  ft.    oin. 
B.    64  ft.    3  in- 
D.    23  ft.  10%  in. 

8,480       17,401 

21.0 

-4 

90.2 

1.6 

Cruiser 
Brooklyn, 

L.  400  ft.    6  in. 
B.    64  ft.    oin. 
D.    2  1  ft.  loJSin. 

8,150       18,770 

21.9 

14    !  269.1       2.26 

Cruiser 
Columbia, 

L.  41  1  ft.    7Kin- 
B.    58  ft.    2%  in. 

7,350       18,509 

22.8 

18 

166.3        0.73 

D.    22ft.    5  in. 

Battleship 
Massachusetts, 

L.  348  ft-    o  in. 
B.    69  ft.    oin. 

10,265      Io»4°3 

16.21 

10 

107.2 

o-99 

ID.    24ft.    6  in. 

Battleship                L"  348ft"  oin' 
Indiana, 

10,225 

9,733  ' 

15-55 

10 

52.8 

0.96 

D.    24  It.    oin. 

L.  360  ft.    oin. 

Battleship  Iowa, 

B.    72ft.    2&in. 

".363 

12,105 

17.09 

8      i    104.6        0.99 

D.    24  ft.    oHin. 

1 

288 


MECHANICAL    DRAFT. 


FIG.  53.    TYPES  OF  STURTEVANT  SPECIAL  STEAM  FANS  APPLIED  FOR  FORCED  DRAFT 
ON  VESSELS  OF  U.  S.  NAVY. 


MECHANICAL    DRAFT. 


FIG.  54.    TYPES  OF  STURTEVANT  SPECIAL  STEAM  FANS  APPLIED  FOR  FORCED  DRAFT 
ON  VESSELS  OF  U.  S.  NAVY. 


2go 


MECHANICAL    DRAFT. 


MECHANICAL    DRAFT.  291 

TJ.  S.  Navy.  —  The  importance  of  forced  draft  as  one  of  the  elements  of  suc- 
cess in  the  modern  warship  has  already  been  pointed  out.  By  its  means  it  is 
possible  to  instantly  and  greatly  increase  the  capacity  of  the  boilers,  to  thereby 
make  greater  speed  possible,  and  to  entirely  change  the  aspect  of  an  en- 
gagement. This  increased  boiler  capacity  is  secured  by  the  introduction  of  a 
few  light  and  comparatively  inexpensive  fans  instead  of  greatly  enlarging  the 
boilers  themselves,  for  the  purpose  of  providing  reserve  capacity,  as  would 
otherwise  be  necessary.  The  Navy  Department  of  the  United  States  was 
among  the  first  to  recognize  the  importance  of  such  an  arrangement,  and  early 
called  upon  this  Company  to  design  and  furnish  special  fans  for  draft  production. 
It  being  the  intention  to  use  these  fans  principally  as  a  reserve,  it  was  seen  to 
be  desirable  that  they  should  be  capable  of  operation  at  high  speed,  both  to 
produce  the  required  air  pressure  (of  frequently  4  inches  of  water  at  the  fan) 
and  to  deliver  the  largest  possible  amount  of  air  while  occupying  the  minimum 
of  space.  High  speed  was  obviously  imperative.  This  was  secured  by  de- 
signing the  special  type  of  double-cylindered  upright  enclosed  engine  already 
shown  in  Fig.  32. 

The  types  of  Sturtevant  fans  presented  in  Figs.  53  and  54  are  so  chosen  be- 
cause they  are  representative,  not  because  they  begin  to  cover  all  the  forms 
designed  for  this  special  work.  The  results  of  full-power  forced-draft  contract 
trials  on  the  vessels  for  which  these  particular  fans  were  furnished  are  presented 
in  Table  No.  125,  which  covers  only  a  few  of  the  vessels  of  the  U.  S.  Navy 
equipped  with  Sturtevant  fans.  In  fact,  nearly  400  Sturtevant  fans  have  been 
installed  for  various  uses  on  over  50  vessels  of  the  U.  S.  Navy.  A  comparison 
of  figures  will  show  that  on  an  average  the  horse-power  required  to  drive  the 
fans  is  only  about  0.88  per  cent  of  the  mean  indicated  horse-power  of  all  the 
machinery,  omitting  the  case  of  Cruiser  Brooklyn,  where  it  is  excessively  high. 
The  average  is  only  about  0.75  per  cent. 

U.  S.  S.  Swatara. —  This  vessel  is  equipped  with  Sturtevant  fans  for  the  pro- 
duction of  draft  upon  the  closed  ashpit  system.  The  fans  are  two  in  number, 
each  having  a  wheel  50  inches  in  diameter  by  24  inches  wide,  enclosed  in  a 
steel-plate  case  arranged  so  that  the  air  may  be  discharged  directly  downward. 
Each  fan  is  driven  by  a  direct-connected  5x5  double  enclosed  upright  engine. 
The  results  of  three  tests  of  these  fans  have  been  given  in  Table  No.  114. 

The  fans  were  installed  in  accordance  with  the  designs  of  Mr.  John  C.  Kafer, 
a  general  illustration  of  whose  system,  as  applied  to  a  marine  boiler  and  sup- 
plied with  a  Sturtevant  fan,  is  presented  in  Fig.  55. 

The  ashpits  are  closed  by  means  of  light  iron  doors,  held  in  place  by  buttons 
at  the  corners  and  made  air  tight  by  asbestos  gaskets.  The  air  from  the  blower 


292  MECHANICAL    DRAFT. 

passes  through  the  duct  to  these  closed  ashpits,  whence  it  has  two  means  of 
escape, —  either  through  the  fuel  on  the  grates,  or  through  openings  in  the  dead 
plates  to  the  space  between  the  inner  and  outer  plates  of  the  fire  doors. 

The  operation  of  the  system  may  be  thus  further  described' :  "  As  the  air 
pressure  in  the  space  between  the  inner  and  outer  plates  of  the  furnace  door  is 
from  one-fourth  to  one-half  inch  of  water  greater  than  the  pressure  of  gases  in 
the  furnace,  no  gases  can  escape  from  the  furnace,  as  the  furnace  door  is  air 
packed.  The  air  pressure  between  the  plates  of  furnace  door  being  greater 
than  the  air  in  the  fire  room  or  the  gases  in  the  furnace,  all  leakage  is  from  this 
space  into  the  furnace  and  to  the  fire  room,  and  is  fresh  air,  making  a  better 
combustion  of  gases  in  the  furnace,  at  the  same  time  keeping  the  furnace  door 
and  front  quite  cool. 

"  A  hinged  valve  in  the  air  duct,  which  is  shown  in  the  illustration  as  closed, 
shuts  off  or  regulates  the  supply  of  air  to  each  furnace,  and  is  operated  by  a 
lever  from  the  fire  room.  When  the  valve  is  open,  the  lever  is  fitted  to  lock 
the  closed  furnace  door. 

"This  device  was  first  fitted  to  the  U.  S.  S.  Alliance  in  1886,  and  has  been  in 
constant  use  while  cruising  under  steam,  and  is  satisfactory  in  every  particular. 
In  this  ship  the  cost  of  two  boilers  is  saved, —  using  four  instead  of  six, —  de- 
creasing the  weight  and  space  occupied  by  the  boilers,  enabling  the  ship  to 
carry  about  40  tons  more  coal,  giving  her  increased  endurance,  at  the  same  time 
increasing  her  maximum  power  over  50  per  cent  above  the  power  with  six 
boilers  under  natural  draft.  Since  the  U.  S.  Steamers  Swatara  and  Kearsarge 
have  been  similarly  fitted  [in  both  cases  with  special  Sturtevant  fans  driven  by 
direct-connected  upright  engines],  and  are  now  cruising  with  two  less  boilers, 
increased  coal  capacity  and  50  per  cent  more  horse-power. 

"  In  a  test  of  the  boilers  of  the  U.  S.  S.  Swatara  by  a  board  of  naval  engineers. 
an  average  air  pressure  of  5  inches  of  water  in  the  air  duct,  with  an  average 
consumption  of  44.6  pounds  of  anthracite  coal  per  square  foot  of  grate  per  hour, 
was  maintained  for  6  hours;  the  maximum  rate  of  combustion  was  52  pounds 
per  square  foot  of  grate.  The  evaporative  efficiency  of  the  boiler  under  forced 
draft  during  a  1 2-hour  test  was  9.68  pounds  of  water  per  pound  of  combustible. 
The  boilers  of  the  Swatara  are  cylindrical,  with  return  fire  tubes  over  the  fur- 
naces 9  feet  in  diameter,  9  feet  long,  with  two  furnaces  in  each  boiler  34  inches 
diameter,  having  grates  5^  feet  long;  the  proportion  of  heating  to  grate  sur- 
face is  23  to  i, —  a  very  low  proportion  for  forced  draft,  the  boilers  having  been 
designed  for  natural  draft  only." 


Kafer  System  of  Forced  Draft.     The  Marine  Journal,  New  York,  March  2, 


MECHANICAL   DRAFT.  293 

American  Line  Pier  14,  N.  R.  International  Navigation  Company,  New 
York,  N.  Y. —  This  plant  serves  as  an  excellent  illustration  of  a  system  of  in- 
duced mechanical  draft  in  connection  with  a  special  device  for  abstracting  the 
heat  from  the  escaping  gases  and  imparting  it  to  the  air  supplied  to  the  boiler 
furnaces.  The  particular  system  here  employed  is  known  as  the  "  Ellis  & 
Eaves,"  is  equipped  with  a  Sturtevant  fan,  and  is  thus  described' :  — 

"This  is  a  combination  of  four  old  principles  :  i.  Induced  (or  suction)  draft; 
2.  'Serve'  tubes;  3.  Retarders  in  tubes;  4.  The  air  heated  by  the  waste 
gases. 

"The  object  is  to  burn  with  safety  and  economy  a  much  larger  quantity  of  coal 
than  has  hitherto  been  done  in  a  boiler  of  a  given  size ;  consequently  to  produce 
the  power  required  with  less  boiler  space,  weight,  and  even  first  cost,  without 
the  troubles  which  frequently  have  accompanied  high  'forced'  draft.  In  the 
'Ellis  &  Eaves'  combination,  fans  are  used  by  preference,  to  exhaust  the 
gases  from  the  boilers.  This  is  no  new  idea,  but  it  has  only  now  been  rendered 
really  practicable  for  high  rates  of  combustion  by  absorbing  the  heat  of  the 
gases  before  they  reach  the  fans.  This  prevents  trouble  of  any  kind  in  the  fans. 
The  fan  power  can  be  increased  to  almost  any  extent,  and  draft  equal  to  that 
of  a  chimney  200,  300  or  more  feet  can  readily  be  thus  obtained.  ...  In 
the  fans  and  fan  engines  used  daily  for  nearly  twelve  months  past  with  Nos.  7 
and  8  marine  boilers  at  the  Atlas  Works,  no  trouble  whatever  has  occurred, 
although  for  at  least  half  the  time  the  combustion  has  been  over  40  pounds  per 
square  foot  of  grate  5  feet  8  inches  long,  exclusive  of  dead  plate.  Spurts  have 
been  made  to  60  pounds  per  square  foot  of  same  size  grate  without  trouble, 
because  the  gases  have  not  exceeded  450°  Fahr.  when  entering  the  fans.  Nos. 
9  and  10  boilers,  now  being  completed,  are  intended  to  be  capable  of  burning  an 
average  of  60  pounds  per  square  foot  of  grate  5  feet  8  inches  long  for  any 
length  of  time,  just  as  Nos.  7  and  8  can  burn  45  pounds  now. 

"  No  one  questions  the  use  of  natural  draft.  This  system  is  natural  draft 
magnified." 

"  The  Serve  tubes,  in  the  first  place,  absorb  a  larger  quantity  of  the  heat  than 
can  be  done  with  plain  tubes.  ...  A  retarder  placed  in  the  centre  of  the 
'  Serve '  tube  (whether  it  be  a  strip  of  flat  iron  or  steel  twisted  into  a  spiral 
shape,  or  a  thin  plain  tube  with  the  fire-box  end  closed)  forces  the  gases  out  of 
the  centre  of  the  tubes  into  the  spaces  between  the  ribs,  and  thus  in  closer  con- 
tact with  the  heat-absorbing  surface  specially  provided.  .  .  .  '  Serve  '  tubes 


i  The  "  Ellis  &  Eaves"  Patent  Combination  Induced  Draft.    Catalogue,  March  i,  1893.    John 
Brown  &  Co.,  Limited,  Sheffield,  Eng. 


294 


MECHANICAL   DRAFT. 


MECHANICAL    DRAFT.  295 

with  retarders  give  at  least  15  per  cent  more  steam,  or  economy,  than  plain 
tubes  of  the  same  diameter  without  retarders,  and  at  least  10  per  cent  more 
than. plain  tubes  of  the  same  diameter.  Whilst  with  ordinary  natural  draft  re- 
tarders cannot  be  used,  they  present  no  difficulty  with  medium  and  high  arti- 
ficial draft. 

"  Heating  the  air  for  the  furnaces  by  the  heat  of  the  waste  gases  serves  the 
double  purpose  of  utilizing  the  latter  for  producing  steam,  and  of  making  them 
harmless  when  they  reach  the  fan,  the  heat  having  been  absorbed.  Vertical 
short  air-heating  tubes  have  been  used  in  another  system  for  this  purpose.  In 
the  '  Ellis  &  Eaves '  system  horizontal  and  long  tubes  are  naturally  more  effi- 
cient, and  so  far  it  is  considered  preferable  to  pass  the  air  through  them, 
instead  of  exhausting  the  gases  through  them. 

"  Summarized  briefly,  the  benefits  of  this  system  are  (taking  natural  draft  and 
plain  tubes  burning  at  15  Ibs.  per  square  foot  of  ordinary  size  grate  as  basis), — 

"  (a)  An  economy  of  10  per  cent  of  fuel  when  burning  at  30  pounds  on  the 
same  size  of  grate,  consequently  with  half  the  number  of  boilers,  the  space  and 
weight  being  less  than  with  the  natural  draft  and  plain  tubes,  and  the  first  cost 
not  more. 

"  (&)  An  equal  amount  of  water  evaporated  per  pound  of  fuel,  when  burn- 
ing at  45  pounds  per  square  foot  of  the  same  size  grate,  consequently  with  one- 
third  the  number  of  boilers ;  the  saving  in  weight,  space  and  first  cost  being 
then  considerable.  In  both  cases  the  amount  of  steam  used  by  the  fans  is 
allowed  for  in  the  comparison  with  natural  draft." 

As  regards  its  other  advantages,  it  is  said1  that  "  the  greater  cleanliness  of  the 
suction  draft  is  another  strong  point  in  its  favor,  the  particles  of  coal  dust  and 
ashes  being  sucked  up  through  the  smokestack  instead  of  being  blown  all  over 
the  ship.  The  fire  room  with  the  Ellis  &  Eaves  draft  is  much  cooler  than  with 
either  natural,  forced  or  closed  stokehold  draft,  so  that  the  firemen  are  always 
in  a  condition  to  do  better  work  and  more  of  it  without  exhaustion. 
Finally,  with  the  Ellis  &  Eaves  draft  there  is  perfect  combustion  and  consequently 
no  smoke,  no  matter  what  description  of  coal  is  used" 

1  "  When  the  power  plant  for  the  recently  constructed  pier  for  the  American 
Line  of  steamships  in  New  York  City  was  being  built,  there  were  installed 
two  single-ended  Scotch  marine  boilers,  both  of  which  were  fitted  with  the 
Ellis  &  Eaves  system  ;  while  one  boiler  was  fitted  with  plain  tubes,  the  other 
was  supplied  with  Serve  patent  ribbed  steel  tubes.  [Fig.  56  shows  the  gen- 
eral manner  in  which  the  plant  is  arranged.]  The  waste  gases  are  carried 


Marine  Journal,  New  York,  June  I,  1895. 


296 


MECHANICAL    DRAFT. 


from  the  breeching  back  over  the  boilers  through  two  air-heating  chambers 
and  finally  pass  from  these  through  the  necessary  connections  to  an  8-foot 
Sturtevant  fan  discharging  into  a  yo-foot  stack.  A  by-pass  around  the  fan  is 
provided  so  that  the  boilers  may  operate  under  natural  draft  if  desired.  The 
air-heating  chambers  contain  a  number  of  3-inch  tubes,  each  about  12  feet 
long,  through  which  the  air  that  is  supplied  the  furnaces  is  drawn.  The  air 

Table  No.  126.  —  Results  of  Tests  of  Ellis  &  Eaves  Induced-Draft    System  with  Stur- 
tevant Fan,  at  American  Line  Pier  14,  N.  R.,  New  York,  N.  Y. 


ITEMS 

Number  and  Conditions  of  Test. 

No.  i. 

No.  2.                 No.  3- 

No.  i  Boiler, 

No.  i  Boiler,     No.  2  Boiler, 

Plain  Tubes 

Plain  Tubes      Serve  Tubes 

with 

with                    with 

Retarders. 

Retarders.          Retarders. 

Date    

1894, 

Sept.  28. 

Oct.  5.         Sept.  29. 

Duration  of  test          .... 

hours, 

6 

6                   6 

Average  steam-gauge  pressure    . 

.     pounds, 

96 

95                  96 

Average  temperature  of  feed  water    . 

.  degrees  Fahr., 

157                 166                 165 

Average  revolutions  of  fan  per  minute, 

507                 472                 468 

Total  water  evaporated 

.    pounds, 

72,661 

59,708            71,697 

Total  coal  burned       .... 

.    pounds, 

I  1  ,000 

8,920              8,715 

Total  combustible  consumed 

.    pounds, 

9.746 

7,780              7,717 

Coal  burned  per  square  foot  of  grate  per 

hour  .    pounds, 

52.08 

42.23              41-26 

Water  evaporated  per  pound  of  coal  from  and  j              ,                    « 

7.274             8.939 

Water  evaporated  per  pound  of   combustible  j                           Q    f 
from  and  at  2  1  20,                                               \  P°«nds,          8.165 

8.332            10.097 

Average  temperature  at  fan  outlet 

.  degrees  Fahr., 

625 

527                 456 

Average  temperature  at  fan  inlet 

.  degrees  Fahr., 

650 

547                 472 

Average  temperature  at  air  down-take 

.  degrees  Fahr., 

400 

347                 3T5 

Average  vacuum  at  fan  inlet 

inches  of  water, 

7-33 

6.61                6.45 

Average  vacuum  over  fires 

inches  of  water, 

3-88 

346                3-56 

Average  vacuum  under  fires 

inches  of  water, 

1.72 

1.38                1.48 

Average  vacuum  at  air  down-take 

inches  of  water,          0.62 

0.58               0.50 

is  highly  heated  in  its  passage  by  the  waste  gases  from  the  boiler,  and  is 
delivered  at  a  high  temperature  both  above  and  below  the  fires,  the  amount 
to  each  being  regulated  by  butterfly  valves.  As  the  fan  tends  to  produce  a 
slight  vacuum  in  the  furnaces,  both  the  ashpit  and  the  fire  doors  are  made 
to  fit  tightly  to  prevent  the  in-leakage  of  cold  air.  To  prevent  cold  air  from 
entering  the  furnace  the  fire  door,  on  being  opened,  automatically  closes  a  large 


MECHANICAL    DRAFT.  297 

butterfly  valve  in  the  uptake,  thus  shutting  off  the  draft.  Retarders,  consist- 
ing of  long  strips  of  thin  steel  of  a  width  equal  to  the  inside  of  the  tube 
diameter,  and  twisted  into  a  helix  of  about  three  turns  in  the  length  of  the 
tube,  are  placed  in  the  boiler  tubes  to  retard  the  flow  of  gases  through  them. 
The  Serve  tubes  also  contain  retarders. 

"  Three  tests  in  all  were  made,  all  of  six  hours'  duration,  the  boilers  in  all 
instances  being  fired  with  No.  i  buckwheat  Susquehanna  coal.  The  first  test 
was  made  with  the  fan  running  at  520  revolutions  per  minute,  this  causing 
a  vacuum  of  7^  to  8  inches  on  the  fan  suction.  All  of  the  hot  air  was  ad- 
mitted to  the  furnaces  underneath  the  grates.  The  boiler  was  fired  lightly 
every  5  minutes,  and  the  fires  were  cleaned  slightly  about  every  45  minutes. 
They  were  three-fourths  cleaned  after  3^  hours'  running.  Tests  Nos.  2  and 
3  were  made  under  the  same  conditions,  the  fan  being  run  at  360  revolutions 
at  the  start  and  gradually  increased  so  that  the  maximum  speed  was  attained 
after  2^  hours'  running.  In  all  of  the  tests  the  thickness  of  the  fires  was 
estimated  at  the  beginning  and  at  the  end  of  the  tests.  The  water  was  meas- 
ured by  a  previously  calibrated  Worthington  meter.  Test  No.  2  was  made 
upon  boiler  No.  i  with  plain  tubes,  and  test  No.  3  upon  boiler  No.  2  fitted 
with  Serve  tubes."  The  general  results  of  the  three  tests  are  presented  in  Table 
No.  126. 

Steamer  L.  C.  Waldo. — This  vessel,  of  the  Roby  Transportation  Company, 
Detroit,  Mich.,  is  equipped  with  the  Ellis  &  Eaves  system  of  induced  draft 
operated  by  Sturtevant  fans.  An  expert  test  of  the  same,  conducted  by  Mr. 
George  C.  Shepard,  is  thus  reported2 :  — 

"  In  the  table  of  performances  of  modern  lake  steamers  in  the  Blue  Book  of 
American  Shipping,  the  tests  of  fourteen  steamers  are  included.  On  only  four 
of  this  number  does  the  coal  consumption  per  horse-power  per  hour  fall  below 
two  pounds.  On  the  other  ten  it  ranges  from  2.02  pounds  to  2.64  pounds,  the 
average  fuel  consumption  being  2.22  pounds  of  coal  per  horse-power  per  hour. 
It  is  almost  needless  to  say  that  none  of  these  ten  steamers  use  artificial  draft. 

"The  test  of  the  L.  C.  Waldo,  published  herewith,  will  be  of  interest  to 
vessel  owners  and  engineers,  as  it  shows  a  consumption  of  1.88  pounds  of  coal 
per  horse-power  per  hour,  including  all  tests, —  but  excluding  one  test,  a  short 
one  made  with  the  dampers  closed,  as  an  experiment.  The  result  is  1.79  pounds, 
or  a  gain  of  some  20  per  cent,  as  compared  with  the  average  of  the  ten  modern 
lake  steamers  above  referred  to. 

'  The  Engineering  Record,  New  York,  Jan.  5,  1895. 
2  Marine  Review,  Cleveland,  O.,  Oct.  22,  1896. 


298 


MECHANICAL    DRAFT. 


"  The  inventors  of  the  Ellis  &  Eaves  system  will  no  doubt  call  attention  to 
the  fact  that  the  Waldo's  boilers  are  equipped  with  plain  tubes  and  not  with 
Serve's  ribbed  tubes,  which  they  claim  is  the  complement  of  their  draft  system, 
and  that  by  their  use  an  additional  10  or  15  per  cent  could  be  obtained.  Allow- 
ing 10  per  cent  for  the  Serve's  tubes,  the  Waldo's  consumption  would  be  1.61 
pounds  ;  and,  allowing  for  auxiliaries,  it  is  not  unreasonable  to  claim  a  consump- 
tion of  only  i  y<z  pounds  for  this  system. 

"The  aggregate  heating  surface  of  the  two  boilers  is  6,230  square  feet  and 
the  aggregate  grate  surface  is  120  square  feet,  making  the  ratio  51.92.  The 
peculiar  feature  in  this  steam  plant  is  to  be  found  in  the  Ellis  &  Eaves  system  of 
induced  draft.  In  this  system  the  products  of  combustion  are  drawn  from  the 
boilers  by  fans. 

Table  No.  127.  — Results  of  Tests  of  Steam    Plant    on    Steamship  L.  C.  Waldo    with 
Ellis  &  Eaves  Induced  Draft  and  Sturtevant  Fans. 


Number  of  Tes 


3        .]            4 

5 

Duration  hours,  minutes,        5:37            5:57 

3:47       5:14 

5:0 

Mean  boiler  pressure        .         .         .        pounds,  j    166           '65.5 

166.8 

166.4 

I67 

Mean  revolutions,  main  engine         .         .         .           80.77        83.48 

82.34 

87.63 

76.2 

Mean  revolutions,  fan  engine  ....         265 

270 

267 

361 

248 

Indicated  horse-power       .        ..        ...         .      1,547.7 

1,748.4 

1,704.2 

2,066.4 

r-352-3 

Heating  surface  per  I.  H.  P.  .         .  square  feet, 

4.02 

3-57 

3.65 

3.01 

4.6 

I.  H.  P.  per  square  foot  of  grate      .         .         .           1  2.9 

14-57 

14.2 

17.22 

11.27 

Speed                          .         .         .    miles  per  hour,         14.21 

J3-3 

13.04 

14.02 

12.04 

Temperature  of  air  entering  furnace,      degrees, 

235 

285 

365       390 

350 

Temperature  of  air  entering  fan       .        degrees, 



405 

530       590 

450 

Draft  at  furnace        .         .         .  inches  of  water, 

0.21 

0.23 

o-5 

0.25 



Draft  at  fan               .         .         .  inches  of  water, 



'•55 

•1.65 

2-5 



Coal  burned  per  hour       .         .         .        pounds, 

2,715 

3-376 

3-7" 

3,546 



Combustible  per  hour      .         .         .        pounds, 

2,370 

2,947 

3,240 

3,095 



Coal  per  I.  H.  P.  per  hour      .         .        pounds, 

'•75 

1.92 

2.17 

1.71 



Coal  per  square  foot  of  grate  per  hour,  pounds, 

22.6 

28.13 

30-9 

29-55 



Coal  per  ton  of  cargo,  per  mile        .        pounds, 



0.063        °-°7  r 

0.063 



Water  evaporated  per  hour      .         .        pounds, 

24,976 

29,782      30,129 

33,990 

22,682 

Water  per  I.  H.  P.  per  hour    .         .        pounds, 

16.13 

17-03 

17.69 

16.45 

16.69 

Water  per  pound  of  coal          .         .        pounds, 

9.2 

8.82 

8.18 

9-52 



Water  per  pound  of  combustible     .        pounds, 

iQ-53 

10.10 

9.29 

10.98 



Water  from  and  at  212°  per  pound  j_         nds 
of  combustible,                                } 

ii-45 

10.91 

10.12 

11.97 



MECHANICAL    DRAFT.  299 

"  In  this  particular  case  the  fans  are  placed,  one  at  the  back  end  of  each  boiler 
and  against  the  bulkhead  between  the  engine  and  the  boiler  rooms."  [The 
arrangement  is  similar  to  that  shown  in  Fig.  56,  in  connection  with  which  the 
general  features  of  the  system  are  presented.  The  general  results  of  the  test 
are  given  in  Table  No.  127.] 

"Test  No.  i  was  made  on  Lake  Huron,  going  up,  with  vessel  drawing  13  feet 
8  inches  aft  and  6  feet  forward,  compartments  aft  full  and  those  forward  3  feet 
deep.  .  .  .  The  vessel  loaded  132,500  bushels  of  wheat  at  Duluth,  equivalent 
to  3,975  net  tons,  and  draft  forward  and  aft  was  14  feet  5  inches  and  14  feet  9 
inches  respectively.  Test  No.  2  was  made  on  Lake  Superior,  coming  down. 
Electric  engine  was  running.  Test  No.  3  was  made  on  Lake  Huron,  coming 
down,  during  daylight.  Dampers  in  ashpit  were  closed.  Test  No.  4  was  made 
on  Lake  Huron,  in  daylight.  Dampers  were  open.  Test  No.  5  was  made  at 
night.  Dampers  were  closed.  .  .  .  Weighed  all  ashes  made  during  Test 
No.  2  and  found  them  to  amount  to  12.7  percent." 

Gordon  Hollow  Blast  Grate. —  One  of  the  devices  for  equably  distributing 
the  air  and  stimulating  combustion  in  connection  with  mechanical  draft  is  the 
hollow  grate  bar.  The  particular  apparatus  here  described,  of  which  a  Sturte- 
vant  blower  forms  an  inherent  part,  "'may  be  resolved  into  three  principal 
parts, —  the  grate  bars  themselves,  lying  just  where  grate  bars  usually  do  ;  the 
main  blast  pipe,  extending  from  just  outside  the  furnace  wall  transversely 
through  the  furnace,  at  right  angles  to  the  grate  bars,  or  the  ground  line,  just  in 
advance  of  the  bridge  wall ;  and  the  connecting  tubes,  extending  vertically  from 
the  main  blast  pipe  below  to  the  grate  bars  above,  and  establishing  connection 
between  the  two.'' 

"  The  blast-grate  bars  are  of  two  kinds  :  i .  The  Combination  Draft  and  Blast 
Grate  bar  for  coal,  coal  refuse,  coke,  bagasse,  tan  bark,  etc.;  2.  The  Tuyere 
grate  bar  for  wood,  sawdust  and  wood  refuse  generally." 

'$/  'HM'-o  "  ^e  Combination  Draft  and  Blast  Grate  bars  .  .  .  may 
be  briefly  described  as  consisting  of  a  perforated  top  [shown  in 
general  view,  Fig.  59]  and  a  hollow  lozenge-shaped  body,  whose  lat- 
erally projecting  angles  are  provided  with  series  of  orifices,  E, 
below  and  registering  with  the  perforations,  D,  in  the  top  of  the 
grate  bar  [see  Fig.  57].  .  .  .  When  the  blast  pressure  is  re- 
moved, the  unobstructed  draft  perforation,  D,  in  the  top  of  the 
grate  performs  its  useful  function,  and  the  natural  draft  prevails." 

'The  Gordon  Patent  Hollow  Blast  Grate,  Catalogue,  16  pp.  Gordon  Hollow  Blast  Grate 
Co.,  Greenville,  Mich. 


300 


MECHANICAL    DRAFT. 


"  The  Tuyere  grate  may  be  described  as  follows  :  An  air  chamber  or  duct 
having  a  transverse  area  3^x6  inches  extends  through  the  body  of  the  grate 
bar.  In  the  heavy  top  of  the  bar  four  or  five  flaring  openings,  each  7  inches  in 
diameter,  are  cast.  These  openings  are  afterwards  bored  and  reamed  to  a  uni- 
form size  for  the  reception  of  the  heavy  lid-shaped  valve  with  which  each  is 
fitted.  As  shown  in  the  accompanying  cut  [Fig.  58],  these  valves  are  each  pro- 

vided with  a  series  of  circumferential 
notches  through  which  the  air  within  the 
bar  escapes  to  fan  the  furnace  fire." 

"  Instead  of  being  laid  side  by  side  in 
58.  the  furnace  like  the  Combination  Draft  and 

Blast  Grate  bars,  the  Tuyere  grate  bars  are  alternated  with  perforated  draft-grate 
bars  of  special  design,  each  of  which  is  likewise  8  inches  wide,  and  of  course  of 
the  same  length  as  the  Tuyere  grate  bars  themselves.  The  reason  for  this  dif- 
ference in  the  arrangement  of  the  Combination  Draft  and  Blast  Grate  bars  and 
the  Tuyere  grate  bars  is,  that  while  the  former  bar  combines  in  itself  the  functions 
of  both  draft  and  blast-grate  bar,  the  latter  is  primarily  designed  as  a  blast- 

grate  bar  only,  so  that  the 
intermediate  draft-grate  bar 
is  required  to  supply  the  nat- 
ural draft." 

"  To  burn  well  a  fire  must 
have  a  sufficiency  of  air. 
Nor  that  alone.  To  be  effec- 
tive, the  air  must  thoroughly 
intermingle  with  the  fuel  par- 
ticles. It  is  because  of  this 
necessity  that  natural  draft 
is  so  inadequate,  and  the 
Gordon  patent  hollow  blast- 

grate  system  so  satisfactory 
saw  ^     ^ 


APPLICATION  OF  STURTEVANT  FAN  TO  GORDON 
HOLLOW  BLAST  GRATES. 


FIG.  59. 

HOT.T.OVV     T?T.AST    fjRATF.S. 

and  similar  fuels  are  to  be 
burned.  Whereas  with  the  former  the  air  can  come  into  contact  with  the 
densely  packed  mass  of  sawdust  or  coal  dust  only  on  the  surface,  so  the  bed  of 
fuel  burns  on  top  alone,  and  at  the  edges ;  with  the  latter,  the  air  is,  through 
hundreds  of  holes,  forced  with  firm  yet  violent  pressure  through  the  mass,  which 
it  thus  loosens  and  thoroughly  permeates,  so  that  everywhere  are  found  dust  and 
air  in  intimate  contact, —  a  condition  most  favorable  to  perfect  combustion." 


MECHANICAL    DRAFT. 


301 


A  positive  supply  of  air  under  considerable  pressure  has  been  found  to  be 
absolutely  essential  to  the  successful  burning  of  bagasse.  In  Fig.  60  is  illus- 
trated a  form  of  bagasse  burner  in  which  the  Gordon  hollow  blast  grates  just 
described  are  employed  in  connection  with  a  Sturtevant  blower.  The  bagasse 
is  brought  to  the  boilers  by  means  of  a  carrier,  whence  it  is  delivered  to  the  fur- 
naces through  hoppers,  as  shown.  The  regular  supply  of  fuel,  the  freedom  from 
admission  of  cold  air  through  frequently  opened  fire  doors,  the  tuyere  form  of 
the  grates  and  the  constant  and  absolute  supply  of  air  under  pressure  combine 
to  produce  the  most  intense  heat.  The  fire-brick  arch  above  the  grates  prevents 
waste  of  heat,  radiates  it  back  upon  the  fire  and  thereby  plays  a  most  important 
part  in  maintaining  a  high  temperature  within  the  furnace  chamber. 


BAGASSE  BURNER  WITH  GORDON  HOLLOW  BLAST  GRATES  AND  STURTEVANT 
BLOWER. 

Gadey  Air  Grate.  —  This  is  another  form  of  hollow  blast  grate  in  connection 
with  which  the  Sturtevant  fans  are  employed.  A  general  view  of  a  boiler 
equipped  with  these  grates  and  a  Sturtevant  blower  is  presented  in  Fig.  61. 

1  "The  Gadey  air  grate  is  composed  of  hollow  cast-iron  grate  bars  2^  inches 
wide,  so  constructed  that  when  they  are  placed  together  to  form  a  grate,  a  uni- 


i  The  Gadey  Air  Grate,  Catalogue,  16  pages,  1896.     Brown  Brothers  Manufacturing  Com- 
pany, Chicago,  111. 


302 


MECHANICAL    DRAFT. 


form  supply  of  air  can  be  injected  into  them  from  a  pressure  blower  and  de- 
livered from  the  interior  of  the  bars  through  slots  to  the  surface  of  the  grate. 
This  compressed  air,  having  its  outlet  at  the  surface  of  the  grates  and  siphoning 
the  natural  draft  between  the  bars,  distributes  its  oxygen  to  every  part  of  the 


FIG.  6t.     BOILER  EQUIPPED  WITH  GADEY  AIR  GRATE  AND  STURTEVANT  BLOWER. 

fuel  on  the  grate  and  creates  rapid  and  perfect  combustion." 

"  In  no  other  way  can  complete  combustion  be  effected  than  by  the  thorough 
penetration  of  oxygen  through  the  mass  of  fuel  on  the  grates.  To  effect  this 
result  with  the  use  of  air  grates,  the  apertures  or  vents  through  which  the  air 


MECHANICAL    DRAFT. 


3°3 


is  delivered  to  the  fuel  should  be  as  close  together  as  possible.  In  the  Gadey 
air  grate  these  vents  are  2^/2  inches  apart  and  extend  longitudinally  almost  the 
entire  length  of  the  bar. 

The  cross-sectional  view,  Fig.  62,  "shows  the  position  of  the  slots  or  air  vents 

in  the  side  of   the  bars  near  the  surface. 

It  will  be  observed  that  the  air  is  forced 
through  the  slot  in  an  oblique  direction,  or 
at  an  angle  of  45  degrees.  When  the  two 
currents  of  air  from  adjoining  bars  meet 
they  create  a  suction  or  siphonage  through 
the  natural  draft  openings  between  the 
bars,  thus  giving  an  increased  volume  of 
air  to  the  fuel.  The  outer  edges  of  the 
bars  overhanging  the  slot  prevent  ashes  or 
clinkers  from  falling  into  the  interior  of  the 
bars  and  form  a  drip  for  slag  and  ashes  to 
drop  into  the  ashpit. 

"The  Gadey  air  grate  is  especially 
adapted  to  the  burning  of  screenings,  saw- 
dust, bagasse  or  any  form  of  refuse  which, 
from  its  fineness  or  tendency  to  pack 
closely  on  the  grates,  is  difficult,  and  in 
fact  impossible,  to  burn  successfully  in 
any  other  kind  of  furnace." 
Cheney  Brothers,  South  Manchester,  Conn.1  — "The  silk  mills  of  Messrs. 
Cheney  Brothers  at  South  Manchester,  Conn.,  have  been  in  active  operation  for 
the  past  fifty  years,  and  at  the  present  time  offer  employment  to  about  2,000  per- 
sons engaged  in  the  spinning,  weaving,  dyeing  and  printing  of  silk  and  velvet 
goods. 

"  The  steam  plant  as  a  whole  is  not  a  new  one,  but  one  which  has  been  ex- 
tended and  improved  from  time  to  time  to  meet  the  growth  of  the  business.  It 
is  a  plant,  however,  in  which  the  owners  fully  recognized  the  advantages  of 
modern  labor-saving  machinery  and  the  devices  that  tend  to  increase  the  eco- 
nomical generation  of  power  About  two  years  ago  the  old  boiler  house  at  the 
lower  mills  was  rebuilt,  and  new  boilers,  coal-handling  machinery,  a  flue  heater 
or  economizer,  and  exhaust  fan  for  handling  the  products  of  combustion  from 
the  boilers,  were  installed." 


FIG.  62.     SECTION  OF  GADEY 
GRATE  BARS. 


i  Boiler  Plant  of  Cheney  Bros.'  Silk  Mills.     The  Engineering  Record,  New  York,  Jan.  6,  1894. 


3°4 


MECHANICAL    DRAFT. 


The  general  arrangement  of  the  induced  draft,  as  designed,  is  presented  in . 
Fig.  63.  The  fan  is  a  special  7x10^  Sturtevant  exhauster,  so  arranged  in 
connection  with  flue  dampers  that  the  gases  may  be  passed  through  the  econo- 
mizer and  the  fan,  as  is  usually  the  case  in  order  to  create  sufficient  draft, 
or  direct  to  the  chimney.  The  fan  was  designed  with  a  special  three-pulley 
arrangement,  so  that  it  could  be  driven  either  by  the  independent  Sturtevant 
engine  or  by  belt  from  the  line  shaft. 


FIG.  63.     INDUCED-DRAFT  PLANT  AS  DESIGNED  FOR  CHENEY  BROTHERS'  SILK  MILLS, 
SOUTH  MANCHESTER,  CONN. 

"The  lower  mills  comprise  buildings  of  a  combined  area  of  110,000  square 
feet,  whose  purposes  require  a  large  amount  of  steam  for  lighting,  power,  heat- 
ing and  dyeing.  For  all  these  purposes  steam  at  55  pounds  pressure  is  gener- 
ated in  four  Babcock  &  Wilcox  boilers  of  1,000  horse-power.  Live  steam  is 
used  in  ceiling  coils  in  cold  weather  to  heat  the  buildings,  while  reducing  valves 
are  used  to  throttle  the  steam  employed  in  the  dyeing  cylinders." 

"The  economizer  contains  480  pipes  5  inches  in  diameter  and  10  feet  long, 
presenting  about  6,000  square  feet  of  heating  surface  against  12,000  square  feet 
in  the  boilers.  As  usually  run,  the  economizer  not  only  heats  the  water  to 
about  212°  Fahr,,  but  supplies  about  50  gallons  per  minute  at  the  same  tem- 
perature to  the  dyehouse.  An  average  of  hourly  readings  during  one  day,  of 


MECHANICAL    DRAFT. 


3°5 


the  temperatures  of  water  and  flue  gases  when  under  the  above  conditions,  gave 
the  following  results,  750  horse-power  being  in  use  "  :  — 

Temperature  water  entering  Berryman  heater  ...  45  degrees. 
Quantity  water  entering  Berryman  heater  per  minute  .  95  gallons. 
Temperature  water  entering  economizer  ....  112  degrees. 
Temperature  water  leaving  economizer  .  .  .  .211  degrees. 
Temperature  flue  gases  leaving  economizer  .  .  .  275  degrees. 

Before  the  economizer  was  installed  the  flue  gases  entered  the  chimney  at 
a  temperature  of  about  475  degrees. 

"The  mechanical  draft  plant  was  introduced  in  this  plant  to  save  the  con- 
struction of  an  expensive  stack,  the  old  stack  being  in  good  order  and  amply 
large  to  discharge  the  gases/ when  aided  by  the  fan.'' 

This  points  clearly  to  the  necessity  of  increased  draft  in  connection  with  an 
economizer.  That  a  fan  is  more  desirable  than  a  chimney  under  these  cir- 
cumstances is  evidenced  by  the  following  statement : '  "  The  gases  are  dis- 
charged through  a  chimney  go  feet  high.  This  chimney  we  used  previous  to 
remodelling  our  plant,  and  while  it  is  higher  than  we  actually  need  to  discharge 
the  gases  from  the  fan,  we  find  it  a  satisfactory  arrangement,  as  it  will  give 
sufficient  draft  to  start  the  fires  and  do  our  night  work  without  using  the  fan. 
As  our  work  is  very  variable,  we  have  never  made  any  accurate  tests,  but  feel 
satisfied  that  the  fan  has  saved  us  the  cost  of  building  a  new  chimney  at  least 
175  feet  high,  and  that  it  is  giving  us  as  satisfactory  results  as  we  could  have 
obtained  by  a  chimney." 

The  Deringer  Colliery  of  the  Cross  Creek  Coal  Company,  Deringer,  Pa.  —  This 
plant,  which  is  illustrated  in  Fig.  64,  consists  of  two  Babcock  &  Wilcox  boilers 
having  an  aggregate  builder's  rating  of  500  horse-power,  equipped  with  a  Coxe 
mechanical  stoker  and  a  No.  8  Sturtevant  "  Monogram  "  blower  for  producing 
the  requisite  draft.  Each  boiler  contains  4,225  square  feet  of  water-heating 
surface,  the  aggregate  grate  area  is  100  square  feet,  the  free  air  space  for 
passage  of  air  amounting  to  20  per  cent  of  the  full  area. 

"  The  furnace,"  as  described  in  detail  by  Mr.  Coxe,2  and  as  herewith  illus- 
trated by  the  accompanying  cut,  Fig.  65,  "consists  essentially  of  a  travelling 
grate  moving  from  the  right  toward  the  left.  The  coal,  which  is  brought  to 
the  hopper  20  by  a  drag,  spout,  or  any  other  convenient  method,  feeds  down  by 


1  Cheney  Brothers,  South  Manchester,  Conn.     Letter  of  Feb.  25,  1896,  to  B.  F.  Sturtevant  Co. 

2  Some  Thoughts  on  the  Economical  Production  of  Steam,  with  Special  Reference  to  the 
Use   of   Cheap    Fuel,  by  a    Miner   of   Coal.     Eckley    B.  Coxe.     Transactions    New   England 
Cotton  Manufacturers'  Association,  April,  1895. 


3o6 


MECHANICAL    DRAFT. 


FIG.  64.     ARRANGEMENT  OF  COXE  MECHANICAL  STOKER  AND  STURTEVANT  FAN  AT 
THE  DKRINGER  COLLIERY  OF  THE  CROSS  CREEK  COAL  COMPANY,  DERINGER,  PA. 


MECHANICAL    DRAFT. 


3°7 


gravity  over  the  fire  brick  14  on  to  the  travelling  grate.  The  coal  is  carried 
slowly  at  the  rate  of  from  3^2  to  5  feet  per  hour  toward  the  other  end.  In  the 
beginning  of  the  operation  the  coal  on  the  right-hand  side  of  the  furnace  is 
ignited,  the  other  part  being  covered  with  ashes  or  partially  consumed  coal. 
After  the  furnace  is  heated,  the  fire  brick  14,  which  we  call  the  '  ignition  brick,' 
becomes  hot,  and  the  coal,  passing  down  under  regulating  gate  21,  becomes 
gradually  heated,  and  by  the  time  it  reaches  the  foot  of  the  ignition  brick  is  in- 
candescent. In  some  cases  the  coal  becomes  hot  enough 
to  ignite  soon  after  it  passes  the  regulating  gate  21. 
Under  the  grate  there  are  a  number  of  chambers  made 


FIG.  65.     ILLUSTRATION  OF  THE 
COXE  PROCESS  AND  FURNACE  FOR  BURNING  SMALL-SIZE  ANTHRACITE  COAL. 

of  sheet  iron  which  are  closed  on  all  sides  except  on  top.  The  blast  from 
the  fan  which  is  used  to  furnish  the  air  is  blown  into  the  large  air  chamber, 
which  is  the  second  one  from  the  right.  These  air  chambers  are  open  on  top, 
but  the  partitions  are  covered  by  plates  27,  28,  29  and  30.  These  plates  are 
of  such  width  that  no  matter  what  may  be  the  position  of  the  grate  bars  18, 
there  is  always  one  resting  upon  this  plate,  so  that  the  air  cannot  pass  from  one 
chamber  to  another  except  by  leakage  along  the  bar.  The  result  of  this  ar- 
rangement is  that  if  we  are  blowing  into  the  large  air  chamber  with  a  pressure, 
say,  of  i-inch  water  gauge,  the  pressure  in  the  next  air  chamber  to  the  left 
would  be  about  ^  inch,  the  next  to  that  yz  inch,  and  the  next  to  that  %  inch. 
Of  course  these  figures  are  not  strictly  correct,  and  are  used  merely  for  the  pur- 
pose of  illustrating,  as  I  am  now  describing  only  the  general  principle  of  the 
apparatus.  The  pressure  in  the  air  chamber  to  the  right  would  be,  say,  3/£  inch. 
The  result  of  this  state  of  affairs  is,  that  the  coal,  when  it  arrives  on  the  grate,  is 


3o8  MECHANICAL    DRAFT. 

subjected  to  a  pressure  of  blast  sufficient  to  ignite  it,  but  not  too  strong  to  im- 
pede ignition.  In  order  to  regulate  exactly  the  pressure  of  the  air  in  each  of 
the  compartments,  the  partitions  are  provided  with  registers,  by  the  simple 
opening  and  closing  of  which  the  pressure  in  the  air  chambers  can  be  varied  to 
suit  the  conditions. 

"As  the  thoroughly  ignited  coal  passes  slowly  over  the  second  compartment 
(where  the  air  pressure  is  a  maximum)  it  burns  briskly,  and  then  slowly  passes 
over  the  third  compartment,  where  the  air  pressure  is  less  and  better  suited  to 
the  combustion  of  the  thinner  layer  of  partly  consumed  coal.  The  bed  continues 
to  diminish  in  carbon  and  to  be  subjected  to  less  blast,  until  finally  the  hot 
ashes  are  cooled  off  (before  being  dumped)  by  a  very  gentle  current  of  air,  which 
is  heated  and  mingles  with  the  carbonic  oxide  produced  in  the  zone  of  intense 
combustion  B,  and  converts  it  into  carbonic  acid ;  the  object  being  to  subject 
the  coal,  as  soon  as  it  arrives  on  the  grate,  to  a  pressure  of  blast  which  is  the 
proper  one  to  ignite  it,  then  to  burn  it  with  a  blast  as  strong  as  will  produce  good 
combustion,  and,  as  the  carbon  is  eliminated  and  the  thickness  of  the  bed 
becomes  smaller,  to  diminish  the  blast  to  correspond  to  these  conditions.  The 
mass  of  coal  remains  all  the  time  in  practically  the  same  position  and  condition 
in  which  it  was  placed  on  the  grate,  except  so  far  as  altered  by  the  combustion." 

"From  this  brief  description  the  continuous  action  of  the  furnace  can  be 
easily  understood.  The  coal,  passing  continuously  down  from  the  ignition 
brick,  is  ignited  gradually,  burned  out,  and  the  ashes  are  carried  off  or  dumped 
by  the  grate  bars  as  they  descend  and  become  vertical." 

The  general  results  of  two  careful  tests  of  this  plant  are  given  in  Table  No. 
128.  The  coal  used  in  each  case  was  (anthracite)  rice.  The  results  of  other 
tests  of  other  small  anthracite  coals  in  connection  with  a  Coxe  stoker  have 
already  been  given  in  Table  No.  46.  It  was  for  the  especial  purpose  of  success- 
fully burning  such  coal  that  this  type  of  furnace  was  designed  and  mechanical 
draft  adopted.  The  general  features  of  the  system  are  thus  stated  by  Mr.  Coxe 
in  the  paper  from  which  quotation  has  just  been  made :  — 

"  We  can  blow  with  dry  air.  It  is  not  necessary  to  use  steam  to  partially  pre- 
vent the  formation  of  clinker,  and  consequently  we  avoid  the  loss  due  to  heating 
the  steam  in  the  fire,  the  waste  of  it  in  producing  the  blast,  and  the  effects  of 
its  decomposition.  While  the  furnace  is  running  regularly  it  produces  at  all 
times  the  same  results ;  that  is  to  say,  you  are  always  evaporating  exactly  the 
same  number  of  pounds  of  water  per  minute,  and  can  therefore  furnish  a  steady 
supply  of  steam.  No  time  is  lost  in  cleaning  fires,  nor  is  cold  air  introduced 
when  shovelling  in  coal,  etc.  If  it  is  desired  to  reduce  the  production  of  steam, 
it  is  only  necessary  to  slow  down  the  engine  so  as  to  reduce  the  blast,  the  speed 


MECHANICAL   DRAFT. 


309 


Table  No.   128.  —  Results  of  Tests  of  Babcock  &  Wilcox  Boilers  with  Coxe  Mechanical 

Stoker  and  Sturtevant  Fan  at  The  Deringer  Colliery  of  The  Cross  Creek 

Coal  Company,  Deringer,  Pa. 


Date  of  test  

Dec.  9,  '96. 

Mar.  1  6,  '96. 

Number  of  boilers          .         .         .         .         . 

Two. 

One. 

Duration  of  test     hours, 

10 

10 

Steam-gauge  pressure     pounds, 

no 

103.8 

Air  pressure  in  ashpit    inches  of  water, 

1.  00 

0.60 

Temperature  of  feed-water   degrees  Fahr., 

45 

42 

Temperature  of  escaping  gases     ....      degrees  Fahr., 

900 

499-5 

Total  dry  coal  consumed        ......        pounds, 

40,692.4 

9,800 

Total  combustion            .......        pounds, 

33.2904 

8,200 

Total  weight  of  water  apparently  evaporated        .         .        pounds, 

277,863 

75.956 

Equivalent  water  evaporated  into  dry  steam  from  and  j                , 
at  2120,                                                                     _  -      (      Pou 

276,853 

92,514 

Equivalent  water  evaporated  per  pound  of  dry  coal    \                , 
from  and  at  2120,                                                             \      Pou 

6.83 

9.44 

Equivalent  water  evaporated  per  pound  of  combustible  ) 
from  and  at  2120,                                                             )      Pounds' 

8.51 

11.28 

Water  evaporated  per  hour  per  square  foot  of  grate  )                , 

276.8 

92.5 

Dry  coal  burned  per  square  foot  of  grate  per  hour       .        pounds, 

20.34 

9.8 

Horse-power  on  basis  of   30  pounds  of   water  evap-  }         , 

orated  per  hour  from  temperature  of    1000  into  > 

802.5 

268.1 

dry  steam  of  70  pounds  gauge  pressure,                    ) 

Per  cent  of  horse-power  developed  above  rating   .... 

60.5 

7.25 

of  the  grate,  and,  if  it  is  desired,  the  thickness  of  the  bed  of  fuel  can  be 
changed.  By  means  of  the  registers  between  the  different  air  compartments  we 
are  able  to  regulate  the  supply  of  air  in  all  parts  of  the  bed  of  fuel  so  as  to  get 
the  best  results,  and  to  control  to  a  large  extent  the  composition  of  our  stack 
gases.  Without  difficulty  we  can  obtain  over  16  per  cent  of  carbonic  acid  and 
less  than  3  per  cent  of  free  oxygen  without  carbonic  oxide,  a  result  hardly  pos- 
sible with  hand  firing ;  at  least  we  have  not  been  able  to  find  such  percentages 
in  any  of  the  published  reports  of  tests.  The  same  thing  applies  to  the  ash. 
We  can,  by  regulating  the  thickness  of  the  bed,  the  speed  of  the  grate,  and  the 
distribution  of  the  air,  reduce  the  ash  as  low  as  is  probably  economical.  We 
have  obtained  ash  very  low  in  carbon,  but  we  think  it  was  at  the  expense  of 
capacity,  and  probably  also  of  heat  in  the  stack  gases  —  too  much  air  being  in- 
troduced. We  can  certainly  burn  the  finer  coals  with  a  less  excess  of  air  than 
we  hoped  for  or  than  is  generally  done.  We  have  shown  that  the  amount  of 
water  evaporated  per  pound  of  coal  does  not  depend  practically  upon  the  size, 
but  only  upon  the  amount  of  heat  units  or  combustible  in  the  coal.  We  have 


3io  MECHANICAL    DRAFT. 

found  that  we  could  use  coal  very  high  in  ash  without  diminishing  very 
materially  the  number  of  pounds  of  water  evaporated  per  pound  of  combustible  ; 
but  we  have  also  found  that  the  purer  the  coal  and  the  larger  the  size,  the 
greater  is  the  capacity  of  the  boiler.  That  is  to  say,  that  we  can  evaporate  more 
water  with  a  given  boiler,  the  purer  the  coal  and  the  larger  the  size ;  although 
the  quantity  of  water  evaporated  per  pound  of  combustible  is  practically  the 
same  in  all  cases,  so  long  as  there  is  no  great  excess  of  dust  in  the  fuel,  which 
in  that  case  stops  up  the  air  passages  between  the  pieces  of  coal  and  thereby 
prevents  a  regular  blast  and  even  burning  of  the  fuel.  We  have  also  found  that 
we  could  burn  bituminous  coal  without  much  difficulty,  and  by  properly  regu- 
lating the  air  avoid  absolutely  the  production  of  smoke.  We  think  that  by  pro- 
viding sufficient  grate  surface  we  can  evaporate  as  much  water,  or  nearly  as 
much,  with  a  ton  of  No.  3  buckwheat  or  rice  coal  as  we  can  with  pea  coal,  pro- 
vided they  are  equally  free  from  impurities  ;  the  only  additional  expense  in  the 
case  of  the  small  coal  being  the  interest  and  depreciation  on  the  additional 
plant  necessary  to  produce  a  given  amount  of  steam." 

Central  Unidad,  Cuba. —  This  is  a  bagasse-burner  plant  consisting  of  two 
batteries  of  two  Babcock  &  Wilcox  boilers,  each  battery  of  an  aggregate  of  856 
rated  horse-power,  being  equipped  with  a  Cook  hot-blast  green-bagasse  burner 
and  a  No.  10  Sturtevant  "Monogram"  fan.  A  view  of  one  of  the  batteries  is 
presented  in  Fig.  66,  while  the  results  of  a  careful  test  are  given  in  Table  No.  129. 

Bagasse  usually  contains  enough  sugar,  if  used  as  fuel,  to  evaporate  the  con- 
tained water.  If,  therefore,  it  can  be  burned  direct  from  the  mill  without  the 
loss  of  the  sugar  due  to  sun  drying,  it  should  give  as  good  results  as  when  dried. 

1  "  Cook's  automatic  apparatus  accomplishes  this  result,  burning  the  bagasse 
automatically  direct  from  the  sugar  mill,  with  a  saving  of  the  large  number  of 
men,  carts  and  oxen  required  for  spreading,  drying,  gathering  and  firing  it  in  a 
dry  state.  It  also  secures  far  better  combustion  than  can  be  had  with  the 
best  hand  firing,  with  no  smoke,  little  refuse  and  a  greatly  increased  evaporative 
capacity.  An  element  of  additional  economy  consists  in  utilizing  the  waste 
heat  escaping  to  the  chimney  for  heating  the  blast.  This  hot  blast  is  peculiarly 
efficient  in  burning  wet  fuel,  because  of  the  greatly  increased  capacity  of  the 
hot  air  for  absorbing  moisture,  and  thus  partially  drying  the  bagasse  before 
burning.  .  .  .  These  considerations  explain  the  fact  that  where  these 
burners  have  been  erected  they  have  always  brought  about  a  large  reduction 
in  the  supplementary  fuel  required  with  dry  bagasse,  besides  giving  more  and 
steadier  steam  pressure.  In  a  well-arranged  plantation  the  bagasse  is  sufficient 
without  other  fuel." 

i  Steam.     Catalogue,  1897.     Babcock  &  Wilcox  Company,  New  York. 


MECHANICAL    DRAFT.  311 

"The  furnace  of  Cook's  apparatus  consists  in  an  oven  of  brick,  having  a 
smaller  chamber  beneath,  into  which  the  blast  previously  heated  is  introduced 
through  numerous  perforations  in  the  walls.  Openings  in  the  walls  of  the  oven 
permit  the  escape  of  the  gases  of  combustion  to  the  boilers.  On  their  way  to 
the  chimney  these  gases  pass  tubular  heaters,  through  which  a  fan  forces  the 
blast  en  route  to  the  burner,  thus  retaining  a  large  part  of  the  waste  heat  to  the 
furnace  and  securing  an  exceedingly  high  temperature  therein.  The  furnaces 


FIG.  66.    COOK'S  BAGASSE  BURNER  WITH  STURTEVANT  FAN,  AT  CENTRAL  UNIDAD,  CUBA. 

require  to  be  cleaned  once  in  24  hours,  when  the  refuse  from  250  tons  of 
bagasse  makes  about  four  wheelbarrow  loads,  in  the  form  of  a  vitreous  mass, 
evidencing  the  intense  heat  attained. 

"  The  bagasse  is  fed  to  the  furnaces  automatically  by  an  arrangement  of  car- 
riers which  receive  it  from  the  rolls  and  distribute  it  equably  to  the  different 
furnaces,  where  more  than  one  is  required,  dumping  any  surplus  upon  cars, 
where  it  is  stored  for  use  when  the  mill  is  not  grinding." 

The  fan,  as  shown  in  the  illustration,  is  driven  by  a  Sturtevant  upright  engine, 
which  thus  renders  the  draft  entirely  independent  of  climatic  conditions  and  of 
any  other  means  of  production. 


312 


MECHANICAL    DRAFT. 


Table  No.  129.  —  Results  of  Test  of  Cook's  Patent  Hot-Blast  Green-Bagasse 
with  Sturtevant  Fan,  at  Central  Unidad,  Cuba. 


Duration  of  test hours, 

Heating  surface  in  boilers          ........    square  feet, 

Average  temperature  of  feed-water degrees  Fahr., 

Average  temperature  of  air  in  heater  chambers  .  .  .  degrees  Fahr.,  ] 
Average  temperature  of  air  in  tuyere  chambers  .  .  .  degrees  Fahr.,  j 
Average  temperature  of  air  in  blast  boxes  ....  degrees  Fahr.,  ! 

Average  temperature  of  atmosphere degrees  Fahr., 

Average  temperature  of  atmosphere  over  heaters     .         ,         .         degrees  Fahr., 
Average  temperature  of  gases  at  base  of  stack         .         .         .         degrees  Fahr.,  ! 
Total  pounds  of  cane  ground    .......... 

Pounds  of  cane  ground  per  hour       ......... 

Pounds  of  cane  ground  to  produce  bagasse  burned  per  hour  .... 

Pounds  of  cane  ground  to  produce  bagasse  for  i  horse-power  per  hour  . 

Total  pounds  of  bagasse  made 

Pounds  of  bagasse  made  per  hour    . 

Total  pounds  of  bagasse  burned  during  test     . 

Pounds  of  bagasse  burned  per  hour          ........ 

Total  pounds  of  bagasse  not  burned,  or  spare  ...... 

Pounds  of  bagasse  not  burned,  or  spare,  per  hour 

Pounds  of  bagasse  burned  to  develop  i  horse-power,  per  hour         .         . 

Pounds  of  juice  extracted 

Average  density  of  juice degrees  Baume, 

Per  cent  extraction    ............ 

Per  cent  bagasse  ........... 

Average  steam  pressure     .........  pounds, 

Average  air  pressure  under  blowers  ......  inches  water, 

Average  air  pressure  in  tuyere  chambers inches  water,  j 

Average  air  pressure  in  blast  boxes  ......  inches  water,  j 

Average  draft  current  in  stack  base inches  water,  j 

Per  cent  dry  solids  in  100  pounds  bagasse 

Per  cent  moisture  in  i  oo  pounds  bagasse  .... 

Per  cent  dry  solids  in  100  pounds  cane  ....... 

Total  pounds  of  water  evaporated  (tank  measurement)     ..... 

Pounds  of  water  evaporated  per  hour 

Pounds  of  water  evaporated  per  square  foot  of  heating  surface  per  hour 
Pounds  of  water  evaporated  per  one  pound  of  bagasse  burned 
Pounds  of  water  evaporated  from  and  at  212°  per  one  pound  of  bagasse  burned, 
Pounds  of  water  evaporated  from  212°  into  steam  of  70  pounds  pressure  per  )    i 

one  pound  of  bagasse  burned,  f   j 

Rated  horse-power  of   boilers  -.   .         .         .       •  ;•    '    *         ...... 

Horse-power  of  boilers  developed  (30  pounds  of  water  evaporated  from  2120  j 

to  steam  of  70  pounds  pressure),  J    I 

Per  cent  above  rated  capacity  


9-75 


'69-35 

5°  5 

233 

235 

82 

92 

468 

745,290 
76,440 

65.532 
44.82 
221,133 
22,680 
189,633 
19,449 
3I>5°° 
3.23° 
'3-3 
524.157 
10.6 
70.32 
29.68 

87 
2.15 

i-55 
1.6 

•35 
53-5 
46-5 
15-87 
408,540 
41,901 
4.26 
2.154 
2.328 

2.245 
856 
1,462 
70 


MECHANICAL   DRAFT.  313 

Table  129.  —  Concluded. 


Horse-power  per  hour  not  used  in  the  spare  bagasse         .      '  .       .  ;    .     . 
Square  feet  of  heating  surface  which  developed  i  horse-power  per  hour 
Quality  of  steam        .         .         .         .         .         .         .         .         .    •     . 

Per  cent  of  ash  on  per  cent  of  bagasse  .  .  ,  :  ~  '.'.".. 
Average  revolutions  per  minute  of  blower  engine  .  •  .  .  .  -'. 
Average  revolutions  per  minute  of  blowers  .  .  . 


242 
6.72 
Dry. 
0.78 
152 


Temperature  of  bagasse  burner  at  7  feet  from  hearth,  indicated 
by  melting  of  copper  rod  ^  inch  diameter, 


degrees  Fahr., 


L.  B.  Darling  Fertilizer  Company,  Pawtucket,  R.  I.  —  A  good  illustration  of 
the  use  of  under-grate  forced  draft  in  the  combustion  of  small  anthracite  coal 
without  special  appliances  other  than  the  introduction  of  a  fan  for  producing 
the  requisite  draft  is  shown  by  the  experience  of  this  concern.  The  boilers, 
which  are  of  the  horizontal  return  tubular  type,  set  in  a  single  battery,  all  sup- 
plied by  the  same  fan,  have  an  aggregate  nominal  rating  of  450  horse-power 
and  a  combined  grate  area  of  1 2 1  square  feet. 

As  stated  in  a  recent  communication,  '"the  coal  used  is  buckwheat,  clear, 
and  costs  $2.87  per  ton  of  2,000  pounds.  The  application  of  the  forced  draft 
is  through  iron  ducts  over  the  top  of  the  boilers,  with  iron  ducts  dropping  to 
each  boiler  at  the  back  end,  and  running  through  the  combustion  chamber, 
to  and  through  the  bridge  wall  to  the  ashpit.  In  the  bridge  wall  is  set  a  blast 
box  54  inches  by  4  inches ;  this  box  is  made  with  a  door,  hinged  at  the  top  to 
swing  to  regulate  the  flow  of  air  to  each  boiler.  Under  ordinary  conditions  the 
rate  of  combustion  is  about  10  pounds  per  square  foot  of  grate  per  hour.  We 
find  the  temperature  of  the  flue  gases  to  be  150°  Fahr.  lower  when  the  blower 
is  in  operation  than  when  it  is  not  in  operation  or  with  natural  draft." 

This  plant  is  equipped  with  a  special  Sturtevant  steel-plate  steam  fan  with 
direct-connected  double  upright  engine,  whose  speed  is  automatically  controlled 
by  a  Burke  regulating  device,  by  means  of  which  the  speed  is  increased  as  the 
steam  pressure  slightly  decreases,  and  vice  versa,  so  that  a  practically  constant 
steam  pressure  is  maintained. 

The  fan  is  located  between  one  end  of  the  battery  of  boilers  and  the  wall  of 
the  building,  occupies  no  valuable  floor  area  and  discharges  the  air  directly 
upward  into  the  horizontal  iron  duct  above  the  boilers.  This  arrangement 
illustrates  the  feasibility  of  introducing  such  a  system  of  draft  production  in  an 
old  boiler  plant. 


iL.  B.   Darling   Fertilizer  Company,  Pawtucket,  R.  I.     Letter  of  Oct.  13,  1897,  to   B.  F. 
Sturtevant  Co. 


MECHANICAL   DRAFT. 


Cleveland  Iron  Mining  Company,  Ishpeming,  Mich.  —  This  plant  serves  as 
an  excellent  illustration  of  the  application  of  a  Sturtevant  blower  in  connec- 
tion with  an  under-feed  mechanical  stoker,  a  type  of  stoker  in  the  operation  of 
which  considerable  draft  pressure,  such  as  can  best  be  produced  by  a  blower,  is 
an  absolute  necessity.  This  plant,  shown  in  Fig.  67,  consists  of  a  battery  of 
72-inch  return  tubular  boilers,  equipped  with  Jones  under-feed  mechanical 
stokers,  No.  7  Sturtevant  "Monogram"  blower  and  a  Sturtevant  upright  engine. 


FIG.  67.    ARRANGEMENT  OF  JONES  UNDER-FEED  MECHANICAL  STOKERS  AND  STURTEVANT 
FAN  AT  CLEVELAND  IRON  MINING  COMPANY,  ISHPEMING,  MICH. 

1 "  The  stoker  consists  of  a  steam  cylinder  or  ram,  with  hopper  for  holding 
the  coal,  outside  the  furnace  proper,  and  a  retort  or  fuel  magazine  inside  the 
furnace,  into  which  the  green  fuel  is  forced  by  means  of  the  ram ;  tuyere  blocks, 
for  the  admission  of  air,  being  placed  on  either  side  thereof ;  the  retort  contain- 
ing at  its  lowest  point,  and  at  a  point  where  the  fire  never  reaches,  an  auxiliary 


'The  Improved  Jones  Under-Feed  Mechanical  Stoker. 
Weeks-Eldred  Company,  Toronto,  Ont. 


Catalogue,  32   pp., 


The 


MECHANICAL    DRAFT.  315 

ram  or  'pusher,'  by  means  of  which  an  even  distribution  of  the  coal  is  obtained. 

"By  means  of  the  rams,  coal  is  forced  underneath  the  fire,  each  charge  of  fuel 

raising  the  preceding  charge  upward,  until  it  reaches  the  fire ;  which  point  it 

does  not  reach  until  it  has  been  thoroughly  coked.     When  in  its  coked  state  it  is 


FIG.  68.      JONES  UNDER-FEED  STOKER. 

forced  upward  into  the  fire.  The  gases  being  liberated  under  the  fire,  and  at 
that  point  mixed  with  air,  must  necessarily  pass  through  the  fire  and  be  con- 
sumed ;  thus  giving  the  benefit  of  all  combustible  matter  in  the  fuel.  Air  is 
forced,  at  a  low  pressure,  through  the  tuyere  blocks,  under  the  burning  fuel,  by 
means  of  a  blower,  operated  by  an  indepen- 
dent engine,  or  from  a  line  shaft,  if  such  ar- 
rangement can  be  made." 

The  stoker  is  shown  in  Fig.  68,  while  Fig. 
69  indicates  the  manner  of  forcing  upward 
and  distributing  the  fuel,  and  the  method  of 
introducing  air,  which  in  this  type  of  stoker  is 
supplied  by  a  Sturtevant  "Monogram"  blower. 

"Coal  being  in  the  hopper,  and  the  ram 
plunger  at  its  forward  stroke,  when  more  coal 
is  needed  the  ram  plunger  is  shifted  by  moA'- 
ing  the  lever ;  coal  then  falls  in  front  of  the 
plunger  and  upon  return  movement  is  forced 
into  the  retort ;  this  movement  being  repeated 
until  sufficient  fuel  is  in  the  retort.  .  .  . 
Air  at  low  pressure  being  admitted  into  the 
air  chamber  and  through  the  tuyere  blocks, 
over  the  top  of  the  green  fuel  in  the  retort, 
but  under  and  through  the  burning  fife!,  the 
result  is  that  the  heat  from  the  burning  fuel 


FIG.  69.     CROSS-SECTION  SHOWING 
FURNACE  IN  OPERATION. 


3i6 


MECHANICAL    DRAFT. 


over  the  retort  slowly  liberates  the  gas  from  the  green  fuel  in  the  retort.  This 
gas,  being  thoroughly  mixed  with  the  incoming  air  before  it  passes  through  the 
burning  fuel  above,  results  in  a  bright,  clear  fare,,  free  from  smoke,  and  the  com- 
plete consumption  of  all  the  heat-producing  elements  in  the  fuel.  The  retort 
being  air-tight  from  below,  and  the  fuel  being  in  a  compact  mass  in  the  retort, 
the  air  will  find  its  way  in  the  direction  of  the  least  resistance,  which  is  up- 
ward, consequently  combustion  takes  place  only  above  the  air  slots ;  hence  the 
castings  of  the  retort  are  always  cool  and  not  subject  to  the  action  of  the  fire. 

Table  No.  130.  —  Results  of  Comparative  Tests  of  Jones  Under-Feed  Mechanical 

Stoker  Equipped  with  Sturtevant  Blower,  and  Hand-Firing,  at  Cleveland 

Iron  Mining  Company,  Ishpeming,  Mich. 


ITEMS. 

Hand-Fired. 

Stoker-Fired. 

Duration  of  test           ....          continuous  hours,                 72 

72 

Average  steam  pressure       .         .         .         .         . 

pounds,              118.95 

118.95 

Average  temperature  of  feed-water    .         .       degrees  Fahr.,  j'            210 

4I.I 

r  .   C/C  . 

Total  ash  and  clinker          .         .         .         .         .     . 

pounds, 

8,245 

8,202-5 

Total  combustible       

pounds, 

42,354 

46,661.5 

Total  water  evaporated       .         *         ... 

pounds, 

312,323 

341,483 

Total  water  evaporated  per  pound  of  coal  at  ob-  \ 
served  temperature,                                           :    .j 

pounds, 

6,176.17 

6,226.22 

Water  evaporated  per  pound  of  coal  from  and  at  ) 

2120,                                                                                                          J 

pounds, 

6-45 

7-57 

Water  evaporated  per  pound  of  combustible  from  ) 
and  at  2  1  20,                                                             J 

pounds, 

7.71 

8.91 

Gain  in  evaporation    .         .         .         .        *. 

per  cent, 

17-37 

The  incoming  fresh  fuel  from  the  retort  forces  the  resulting  ash  and  clinker 
over  the  top  of  the  tuyere  blocks  on  to  the  side  plates,  from  whence  they  may 
be  removed  at  any  time  without  in  the  least  interfering  with  the  fire  in  the  centre 
of  the  furnace,  resulting  in  a  high,  even  temperature  at  all  times." 

In  Table  No.  130  are  given  the  results  of  an  extended  comparative  test,  made 
Oct.  30,  31,  Nov.  i  and  2,  1895,  between  this  stoker  and  hand-firing  at  the 
Cleveland  Iron  Mining  Company,  which  serve  to  show  its  relative  economy. 
In  each  case  the  test  was  made  upon  two  boilers,  and  the  same  kind  of  coal  — 
bituminous  slack  —  was  used.  The  coal  consumed  in  the  stoker-fired  boilers 
includes  the  amount  used  in  a  fifth  boiler  to  make  steam  to  run  the  blower  en- 
gine. The  water  evaporated  in  the  fifth  boiler  is  not  included. 


MECHANICAL    DRAFT.  317 

John  Brown  &  Co.,  Ltd.,  Sheffield,  Eng.  —  The  boiler  to  which  the  following 
description  relates  is  one  of  several  to  which  Sturtevant  fans  are  applied  at  the 
works  of  the  above-named  firm.  It  is  of  novel  construction,  designed  for.the 


FIG.  70.     EAVES  HELICAL  INDUCED-DRAFT  BOILER. 

purpose  of  experimentally  demonstrating  the  merits  of  the  Eaves  helical  in- 
duced-draft system,  which  has  been  subjected  to  long-continued  and  very  careful 
investigation  by  the  above-named  firm. 


3c8  MECHANICAL    DRAFT. 

This  system  is  a  combination  of  mechanical  draft,  Serve  tubes  and  retarders 
and  a  means  of  abstracting  a  portion  of  the  heat  from  the  gases  and  utilizing  it 
in  raising  the  temperature  of  the  air  supplied  to  the  furnaces.  In  connection 
with  Fig.  70  the  following  description1  will  serve  to  make  the  system  clear:  — 

"  The  cold  air  for  the  combustion  of  the  fuel  enters  from  the  back  end  of  the 
boiler,  passing  along  the  outer  space  A  and  A1  to  the  valves  B  and  B1  in 
furnace  fronts ;  on  its  way  this  cold  air  is  guided  round  the  outside  of  the 
'  inner  '  space  C  in  a  '  helical '  direction  by  partitions  set  up  as  shown.  After 
combustion,  the  waste  hot  gases,  leaving  the  boiler,  pass  through  the  smoke-box 
into  inner  space  C  and  are  made  by  similar  partitions  to  pass  round  and  in 
close  contact  with  boiler  in  a '  helical '  direction  on  their  way  to  the  suction  fan. 

"The  boiler  by  these  means  is  thoroughly  enveloped  in  the  escaping  heat, 
effectually  preventing  either  radiation,  condensation  or  straining  of  the  boiler 
under  any  forced  conditions,  such  as  rapid  generation  of  steam  from  cold  water 
or  sudden  and  greatly  increased  evaporation.  The  cold  air  on  its  way  to  the 
valves  also  absorbs  a  large  amount  of  heat  from  the  escaping  gases  and  so 
enters  the  furnaces  at  a  greatly  increased  temperature,  with  resultant  economy. 

"  No  blocking  up  of  the  bottom  boiler  tubes  through  any  deposit  in  the 
smoke-box  can  take  place,  as  such  deposit,  if  any,  drops  to  the  bottom  of  inner 
casing  C,  from  whence  it  is  easily  removed  by  doors  at  front.  The  doors  D  are 
placed  so  as  to  allow  of  a  brush  being  passed  through,  to  sweep  away  any  sooty 
deposit  from  the  inner  boiler  shell,  should  any  such  deposit  take  place. 

"  Referring  to  the  annexed  trials  [see  Table  No.  131.    This  boiler  was  equipped 
with  a  Sturtevant  fan  with  special  cooling  device  to  permit  of  the  handling 
of  the  high-temperature  gases],  we  find  that  the  boiler  efficiency  in  one  case  was 
82  per  cent  and  in  the  other  78  per  cent  of  the  actual  calorific  value  of  the 
coal  used.     If  we  take  the  mean  of  these  figures, —  namely,  80  per  cent, —  and 
work  out  the  evaporation  on  the  basis  of  best  Welsh  coal,  we  obtain  the  follow- 
ing remarkable  results  :  — 

Heat  units  from  combustion  of  one  pound  of  best 

Welsh  coal      .         .         .         .         .         .         .      15,629 

Latent  heat  of  evaporation  from  and  at  212°  .         .  966    t 

Calorific  value  of  coal  in  pounds  of  water  evapo- 
rated per  pound  of  coal  from  and  at  212°,  -^Vz^  =  J6.i8 

966 

16.18  X  80 
oo  per  cent  ot  above  calorific  value,  —  I2-9S  pounds. 


i  Eaves  Helical  Induced  Draft.     Catalogue,  October,  1896.     John  Brown  &  Co.,  Ltd.,  Shef- 
field, Eng. 


MECHANICAL   DRAFT.  319 

"Or  practically  the  evaporation  of  13  pounds  of  water  per  pound  of  coal  from 
and  at  212°  Fahr.,  with  a  rate  of  combustion  of  over  30  pounds  of  coal  per 
square  foot  of  grate  and  a  ratio  of  heating  surface  to  grate  of  only  28  to  i. 

Table  No.   131.  — Results  of  Test  of  Eaves'  Helical  Draft  Boiler  with  Sturtevant  Fan. 


Duration    hours, 

7                               7 

Total  coal  burned      pounds, 

7,58l                                  8,022 

Total  water  evaporated     .....          pounds, 

71,000                              71,500 

Temperature  of  feed-water        .         .         .        degrees  Fahr., 

54                          5° 

Steam  pressure          .         .         .          pounds  per  square  inch, 

43-4                          45 

Revolutions  of  fan  engine  per  minute       .         .         .         . 

508                         520 

Temperature  of  air  at  side  valve       .         .        degrees  Fahr., 

234                        259.5 

Temperature  of  gases  at  inlet  of  fan         .        degrees  Fahr., 

309 

353-8 

Temperature  of  gases  in  smoke  box          .         .         .         .    •: 

Melted  bismuth, 
not  lead. 

Melted  bismuth, 
not  lead. 

Vacuum  under  grate  bars           .         .         .     inches  of  water, 

0-75 

0.64 

Vacuum  over  fires      inches  of  water, 

0.82 

0.8  1 

Vacuum  at  base  of  chimney       .         .         .     inches  of  water, 

4-59 

4-59 

Vacuum  above  fan  outlet            .         .         .     inches  of  water, 

0.38 

0.38 

Velocity  of  air  per  minute  under  grate  bars       .         .       feet, 

1,476 

1,362 

Velocity  of  air  per  minute  through  outer  casing        .       feet, 

257 

227 

Temperature  of  the  air  entering  outer  casing,  degrees  Fahr., 

78 

67 

Coal  burned  per  square  foot  of  grate  per  hour,          pounds, 

33-84 

35-8i 

Water  evaporated  per  pound  of  coal         .         .          pounds, 

9-36 

8.91 

Water  evaporated  per  pound  of  coal  from  and  at  }      ouncjs 

II.  12 

10.63 

Calorific  value  of  coal  used,  expressed  in  pounds  water  ) 
evaporated  from  and  at  2  1  20,                                             j 

I3.6 

13.6 

Efficiency  of  boiler    ......        per  cent, 

82 

78.3 

"To  obtain  an  equally  high  evaporative  efficiency  with  a  Lancashire  boiler, 
it  would  require  to  be  fitted  with  an  economizer  having  a  combined  total  ratio 
of  heating  surface  to  grate  surface  of  75  to  i,  with  a  rate  of  combustion  of 
only  12  pounds  of  coal  per  square  foot  of  grate.  Therefore,  to  produce  the 
same  amount  of  steam  per  hour, — namely,  10,200  pounds,  —  two  Lancashire 
boilers  would  be  required,  8  feet  in  diameter  by  30  feet  long,  with  their  attend- 
ant economizer,  the  floor  space  occupied  being  four  times  that  required  for  the 
helical  draft  boiler. 

"  An  ordinary  marine  boiler  of  the  same  dimensions  as  the  one  used  in  these 
trials  —  namely,  10  feet  6  inches  diameter,  by  10  feet  6  inches  long — will  with 
good  natural  draft  evaporate  about  5,000  pounds  of  water  per  hour;  the  effi- 


320 


MECHANICAL   DRAFT. 


ciency  being  about  65  per  cent,  or  equal  to  10.5  pounds  of  water  evaporated  per 
pound  of  best  Welsh  coal  from  and  at  212°,  instead  of  13  pounds.  From  this  it 
follows  that  two  boilers  with  natural  draft  would  be  required  to  do  the  work  of 
one  of  same  size  fitted  with  the  helical  draft  and  '  Serve '  tubes,  the  relative 
efficiency  being  as  65  to  80. 

"  In  addition  to  the  saving  in  space  effected  by  this  system  of  draft  when  ap- 
plied to  marine  boilers,  the  saving  in  weight  is  also  very  great,  when  we  consider 
that  by  the  addition  of  about  6  tons  to  the  weight  of  a  boiler  installation  of 
t  the  size  under  consideration  —  to  cover  the  weight  of  the  fan  and  en- 

gine, helical  casing,  and  greater  weight  of  Serve  tubes  —  the  power 
of  the  boiler  can  be  doubled,  at  the  same  time  effecting  a  saving  in 
coal  consumption  compared  with  two  ordinary  boilers  of  about  2 '4 
tons  per  24  hours." 

I 


ARRANGEMENT  OF  STURTEVANT  FANS  FOR  MECHANICAL  DRAFT  AND  AIR 
HEATING  AT  UNION  TRACTION  COMPANY,  PHILADELPHIA,  PA. 

Union  Traction  Company,  Philadelphia,  Pa.  —  The  complete  boiler  plant  con- 
sists of  three  horizontal  return  tubular  boilers  60  inches  in  diameter  by  1 7  feet 
8  inches  long  over  all,  and  is  employed  for  the  generation  of  steam  at  about  15 
pounds  pressure  for  heating  the  car  sheds,  shops  and  offices  covering  an  entire 
square.  For  utilizing  the  waste  heat  in  the  flue  gases,  an  air  heater  is  inter- 
posed between  the  boiler  uptake  and  the  stack.  Through  the  longitudinal 
tubes  of  this  heater  the  hot  gases  pass  ;  while  air  admitted  to  the  bottom  of 
the  cylindrical  shell  circulates  around  the  tubes  and  is  discharged  at  the  top. 
To  secure  the  necessary  draft  to  overcome  the  resistance  of  the  air  heater,  and 
at  the  same  time  render  the  draft  positive  and  independent  of  climatic  condi- 
tions, a  loo-inch  Sturtevant  fan  is  provided,  through  which  the  gases  are  drawn 
and  discharged  into  the  vertical  stack  extending  from  the  outlet  of  the  fan. 
This  fan  is  driven  by  belt  from  an  independent  electric  motor. 


MECHANICAL   DRAFT.  321 

To  secure  a  positive  and  sufficient  movement  of  air  through  the  heater, 
another  Sturtevant  fan  —  a  yo-inch  steel-plate  exhauster  —  is  employed.  This  is 
driven  by  a  direct-connected  Sturtevant  electric  motor.  Both  fans  are  clearly 
shown  in  the  view  presented  in  Fig.  72,  while  the  general  arrangement  of  the 
entire  plant  is  indicated  in  Fig.  71.  Economy  of  space  and  avoidance  of  in- 


FIG.  72.     STURTEVANT  FANS  FOR  MECHANICAL  DRAFT  AND  AIR  HEATING  AT  UNION 
TRACTION  COMPANY,  PHILADELPHIA,  PA. 


322 


MECHANICAL   DRAFT. 


FIG.  73.    STEAM-PRESSURE  CHART  FROM  INDUCED-DRAFT   PLANT  AT  UNION   TRACTION 
COMPANY,  PHILADELPHIA,  PA. 


MECHANICAL  DRAFT. 


323 


FIG.  74.    AIR-TEMPERATURE  CHART  FROM  AIR  HEATER  AT  UNION  TRACTION  COMPANY, 
PHILADELPHIA,  PA. 


324 


MECHANICAL    DRAFT. 


direct  passages  for  air  and  gases  are  secured  by  placing  the  induced-draft  fan 
upon  a  platform  above  the  heating  fan.  The  hot  air,  leaving  the  end  of  the 
heater,  is  conducted  through  galvanized  iron  pipes  to  various  parts  of  the  build- 
ings, and  there  properly  distributed.  A  suitable  arrangement  of  dampers  makes 
it  possible  to  shut  any  desired  connection  at  will.  The  one  which  is  located 
near  the  inlet  of  the  induced-draft  fan  is  operated  by  an  automatic  damper 
regulator. 

The  coal  used  is  buckwheat,  of  which  from  6  to  8  tons  are  used  every  24 
hours.  The  actual  results  secured  as  regards  regulation  of  steam  pressure  and 
temperature  to  which  air  is  heated  by  passing  through  the  air  heater,  are  very 
clearly  shown  in  Figs.  73  and  74,  which  are  reproduced  directly  from  charts 
taken  respectively  from  the  recording  pressure  gauge  and  the  recording  ther- 
mometer under  ordinary  working  conditions.  The  former  serves  to  show  that 
the  maximum  variation  in  steam  pressure  does  not  exceed  two  pounds.  The 
latter  demonstrates  the  relative  uniformity  maintained  in  temperature  of  flue 
gases,  of  which  the  temperature  of  the  heated  air  is  to  a  certain  extent  an 
index,  although  influenced  not  only  by  the  temperature,  but  also  by  the  velocity 
of  the  flue  gases  passing  through  the  air  heater. 

Dust  Destructors  at  Shoreditch,  London,  Eng.  —  The  complete  and  satisfactory 
destruction  of  dust,  garbage  and  similiar  refuse  material  is  rapidly  becoming 
one  of  the  most  important  elements  in  modern  sanitary  engineering.  In  a 
large  number  of  the  successful  furnaces  constructed  for  this  purpose,  it  has  been 
found  necessary  to  secure  the  required  intensity  of  draft  and  temperature  by 
forcing  the  combustion  by  artificial  means.  For  this  purpose  the  Sturtevant 
fan  has  proved  itself  readily  adaptable. 

'"The  vestry  and  parish  of  St.  Leonard's,  Shoreditch,  has  proved  itself  to  be 
extremely  enterprising,  and  is  building  upon  a  very  central  site  a  combination  of 
municipal  undertakings.  Of  these  perhaps  the  most  important  is  the  electricity 
and  dust-destruction  undertaking,  which  was  formally  opened  on  Monday,  the 
28th  June,  by  Lord  Kelvin,  in  the  presence  of  a  numerous  gathering." 

After  finally  acquiring  the  site,  "  the  lighting  committee  forthwith  instructed 
Messrs.  Manlove,  Olliott  &  Co.,  Ltd.,  to  report  as  to  what  could  be  done  with 
the  type  of  dust  destructor  manufactured  by  them,  and  what  results  they  could 
guarantee  to  obtain  by  burning  20,000  tons  of  refuse  per  annum.  The  firm  re- 
ported that  with  the  thermal  storage  system  of  Mr.  Druitt  Halpin  such  refuse 
could  produce  sufficient  heat  for  the  electric  lighting  station  proposed  by  Mr. 
Manville,  and  the  value  of  the  steam  so  produced  would  be  ,£4,290  per  annum, 


i  Electricity  and  Dust  Destruction  in  Shoreditch.     The  Engineer,  London,  July  2,  1897. 


MECHANICAL    DRAFT. 


325 


and  that  a  saving  of  at  least  .£1,500  per  annum  could  be  effected  by  burning 
refuse  instead  of  barging  it  away,  as  was  then  being  done.  .  .  .  Formerly 
they  had  to  pay  3^.  zd.  per  ton  of  refuse  for  barging  it  away,  but  now  it  would 
only  cost  is.  2d.  per  ton  for  burning  it  in  the  dust  destructor,  —  an  obvious 
saving  of  2S.  per  ton." 


FIG.  75.     PLAN  OF  DESTRUCTOR  HOUSE,  SHOWING  STURTEVANT  FANS  AT  SHOREDITCH, 

LONDON,  ENG. 

"The  destructor  house  [see  Fig.  75]  is  80  feet  square,  and  contains  12  cells, 
each  having  25  square  feet  grate  area,  and  6  water-tube  boilers,  each  with  1,300 
square  feet  of  heating  surface.  The  boilers  and  thermal  storage  vessel  —  which 
is  35  feet  long  and  8  feet  in  diameter— are  designed  to  work  at  a  pressure  of 
200  pounds  per  square  inch,  and  are  supplied  with  duplicate  fittings  throughout 
to  guard  against  breakdown.  There  are  3  [Sturtevant]  motor-driven  fans  cal- 
culated to  deliver  each  8,000  cubic  feet  of  air  per  minute  with  a  maximum  ashpit 
pressure  of  3  inches  of  water." 


326 


MECHANICAL    DRAFT. 


MECHANICAL    DRAFT.  327 

The  general  arrangement  of  the  three  Sturtevant  fans,  which  per  the  above 
are  intended  to  work  at  high  pressure,  is  clearly  shown  in  Fig.  75.  They  are  all 
of  the  downward-discharge  type,  delivering  the  air  to  underground  ducts  ex- 
tending beneath  them,  and  hence  distributing  it  to  the  destructor  cells.  The 
inlets  of  the  fans  are  arranged  to  draw  their  air  supply  from  the  main  sewer, 
thereby  ventilating  the  same  for  a  considerable  distance  in  the  neighborhood  of 
the  destructor  works.  Connection  is  also  made  to  the  inlet  trunks  from  the 
space  immediately  above  the  cells,  which  in  hot  weather  becomes  almost  unbear- 
able for  the  men  employed  in  dumping  the  rubbish,  unless  the  fans  are  used  to 
draw  the  hot  air  away.  In  Fig.  76  (from  "The  Engineer")  is  shown  the  front  of 
the  battery  of  destructor  cells  and  boilers;  the  fans  being  located  in  the  room 
to  which  the  door  at  the  farther  end  of  the  boiler-room  admits. 

The  economy  and  utility  of  forced  draft  are  shown  in  this  connection  by  the 
fact  that  the  fans  were  proportioned  on  the  estimated  requirement  of  only  190 
cubic  feet  of  air  per  pound  of  refuse,  and  a  rate  of  combustion  of  26  pounds  of 
refuse  per  square  foot  of  grate  per  hour. 

United  States  Cotton  Company,  Central  Falls,  R.  I.  —  This  is  the  plant  already 
referred  to  in  Chapter  V.  as  showing  a  weekly  saving  of  over  $126.00  resulting 
from  the  introduction  of  mechanical  draft.  It  was  there  shown  that  this  result 
was  principally  due  to  the  burning  of  cheap  fuel,  which  was  only  made  possible 
by  the  introduction  of  the  fan.  There  is  here  presented  the  record  of  two  con- 
secutive tests  of  13  weeks'  duration  each,  showing  economic  results  that  are 
worthy  of  the  closest  attention,  and  agreeing  with  remarkable  closeness  as  to 
the  exact  average  cost  per  indicated  horse-power  for  the  entire  period. 

'"This  boiler  plant  consists  of  a  battery  of  three  Babcock  &  Wilcox  water- 
tube  boilers,  each  18  tubes  wide  and  9  tubes  high,  with  three  steam  drums  36 
inches  in  diameter.  The  furnaces  are  10  feet  10  inches  by  7  feet.  These  boilers 
are  equipped  with  automatically  controlled  mechanical  draft,  air  being  supplied 
to  the  ashpits  through  a  blast  box  60  inches  by  5  inches  in  the  bridge  wall  of 
each  boiler,  by  means  of  a  special  Sturtevant  go-inch  fan,  driven  by  a  direct- 
connected  cross-compound  4-7x4  upright  enclosed  engine.  The  steam  pressure 
carried  on  this  plant  is  150  pounds.  The  engine  is  a  cross-compound  con- 
densing of  the  Harris-Corliss  type. 

"During  thirteen  weeks,  ending  April  3,  1897,  the  engine  ran  up  to  speed 
773.94  hours,  developing  an  average  of  1,562.36  indicated  horse-power,  as 
shown  by  cards  taken  twice  per  day.  All  the  fuel  fired  to  the  furnaces,  for 


i  Thomas  P.  Burke,  chief  engineer  of  United  States  Cotton  Company,  Central  Falls,  R.  I. 
Letter  of  Oct.  6,  1897,  to  B.  F.  Sturtevant  Co. 


328 


MECHANICAL   DRAFT. 


MECHANICAL    DRAFT.  329 

banking  fires  at  night  and  getting  up  steam  in  the  morning,  is  charged  to  the 
time  and  horse-power  of  the  main  engine,  as  follows :  — 

Weight  in  Pounds.  Cost. 

Dry  buckwheat  coal,  containing  an  average  of 

17%  of    dust  or  anthracite  culm  that  would 

pass  through  a  TVinch  square-mesh  screen     .  1,613,127  52.019.91 

Dry  dust  or  anthracite  culm      ....  287,781  215.48 

Low-grade  Cumberland     .          .          .          .          .  170,983  313-11 

Total 2,071,891  $2,548.50 

Making  a  cost  of of  a  cent  per  indicated  horse-power  per  hour.      The 

1,000 

ashes  and  unconsumed  coal  taken  from  the  furnaces  and  ashpits  at  night  aver- 
aged 12.86  per  cent. 

For  the  thirteen  weeks  ending  Oct.  2,  1897,  the  conditions  were  substantially 
the  same.  The  engine  ran  to  speed  708.94  hours,  developing  an  average  of 
1,545.24  indicated  horse-power.  The  fuel  fired  to  the  furnaces  was  — 

Weight  in  Pounds.  Cost. 

Dry  buckwheat  coal  containing  18%  of  dust 

or  anthracite  culm         .....  1,518,549  $1,927.50 

Dry  dust  or  anthracite  culm      .          .          .          .  253,253  190.90 

Low-grade  Cumberland 94,934  142.51 

Total 1,866,736  $2,260.91 

Making  a  cost    of  of  a  cent   per  indicated  horse-power  per  hour.     The 

1,000 

ashes  and  unconsumed  coal  taken  from  the  furnaces  and  ashpits  was  12.9  per 
cent. 

"  The  temperature  of  the  flue  gases  directly  after  leaving  the  boiler  is  from 
380°  Fahr.  to  415°  Fahr.  The  blast  pressure  in  the  air  duct  is  from  zero,  when 
the  fan  is  running  very  slowly,  to  1.3  inches  of  water  when  it  is  speeded  up. 
The  average  evaporation  from  and  at  212°  per  pound  of  combustible  was  10.28 
pounds,  metered  through  a  Worthington  hot- water  meter,  calibrated,  and  allow- 
ance made  for  slip.  During  part  of  the  thirteen  weeks,  ending  April  3,  the 
meter  was  out  of  commission  for  repairs,  so  no  evaporation  is  given.  By  the 
term  '  dry  coal '  is  meant  coal  atmospherically  dry ;  that  is,  dried  in  the  open 
air." 

The  general  results  of  these  continuous  tests  are  presented  in  Table  No.  132. 
From  the  hourly  cost  per  indicated  horse-power  there  given  it  is  evident  that 
the  fuel  cost  per  year  per  indicated  horse-power  (based  on  58  working  hours  per 


33° 


MECHANICAL    DRAFT. 


FIG.  78.     STEAM-PRESSURE  CHART  FROM  FORCED-DRAFT   PLANT  AT  UNITED   STATES 
COTTON  COMPANY,  CENTRAL  FALLS,  R.  I. 


MECHANICAL    DRAFT. 


week  and  52  weeks  per  year)  is  respectively  only  $6.33  and  $6.21,  as  indicated 
by  the  results  of  the  two  tests. 

Table  No.  132. —  Results  of  Two  Thirteen-Week  Tests  of  Steam  Plant  with  Sturtevant 
Mechanical  Draft,  at  United  States  Cotton  Company,  Central  Falls,  R.  I. 


ITEMS. 

Test  No.  i. 

Test  No.  2. 

Duration         ........ 

hours,               773.94 

708.94 

Average  steam  pressure         ..... 

pounds,              1  50 

I50 

Average  I.  H.  P.  of  compound  engine 

1,562.36                   1,545-24 

Total  fuel  fired      

pounds,           1,613,127 

1,518,549 

Ash  and  unconsumed  coal     

per  cent,                12.86 

I  2.9 

Water  evaporated  from  and  at  2120  per  pound  of 
combustible, 

•  pounds, 

10.28 

Total  cost  of  fuel          

$2,548.50 

$2,260.91 

Cost  of  fuel  per  hour  for  one  I.  II.  P. 

$0.0021 

$O.O0206 

The  front  of  a  portion  of  the  battery  of  boilers,  together  with  the  Sturtevant 
fan,  is  clearly  shown  in  Fig.  77.  The  fan  is  of  the  angular  down-blast  pattern, 
the  air  being  discharged  into  an  underground  duct  extending  beneath  the 
boilers.  The  air  supply  for  the  fan  is  drawn  from  above  the  boilers  through  the 
pipe  shown  in  the  illustration.  This  serves  to  keep  the  upper  part  of  the  boiler 
house  cool  and  well  ventilated.  The  perfection  of  steam-pressure  regulation 
secured  by  the  use  of  the  fan  is  most  emphatically  shown  by  the  steam- 
pressure  record,  reproduced  in  Fig.  78.  This  result  is  largely  due  to  the 
method  of  regulation  of  the  speed  of  this  engine.  This  is  accomplished  by 
means  of  a  device  especially  designed  and  patented  by  the  engineer,  Mr. 
Thomas  P.  Burke,  which  instantly  changes  the  speed  of  the  fan  to  correspond 
with  very  slight  variations  in  the  steam  pressure. 

Glens  Falls  Paper  Mill  Company,  Fort  Edward,  N.  Y.  —  The  plant  shown 
in  Figs.  79  and  80  serves  as  an  excellent  illustration  of  the  manner  in  which 
under-grate  forced  draft  can  be  applied  in  an  existing  plant.  The  boilers  are 
eleven  in  number,  72  inches  in  diameter  by  19  feet  long  over  all,  arranged  in  five 
batteries,  four  of  these  having  two  boilers  each  and  one  having  three  boilers. 
A  special  7x4  Sturtevant  steel-plate  steam  fan,  located  in  a  room  opposite  the 
centre  of  the  row  of  boilers,  delivers  the  air  into  a  pipe  which  passes  upward  at 
an  angle,  and  thence  over  to  a  point  above  the  boilers,  where  it  divides  and 
extends  the  entire  length  of  the  plant.  From  this  pipe  branches  are  carried 
down  between  the  groups  of  boilers  and  enter  the  ashpits  at  the  sides,  each 
pipe  being  provided  with  a  blast  grate. 


33  2 


MECHANICAL    DRAFT, 


FIGS.  79  AND  80.     FORCED-DRAFT  PLANT  AT  GLENS  FALLS  PAPER  MILL, 
FORT  EDWARD,  N.  Y. 


MECHANICAL    DRAFT. 


333 


Crane  &  Breed  Manufacturing  Company,  Cincinnati,  Ohio.  —  One  type  of  un- 
der-feed mechanical  stoker  used  in  connection  with  a  Sturtevant  fan  has  already 
been  described.  In  Fig.  81  is  shown  a  plant  equipped  with  the  American  Stokers. 
A  No.  5  Sturtevant  "Monogram"  blower,  driven  by  a  4x4  Sturtevant  upright 
engine,  serves  to  supply  the  requisite  air  under  the  pressure  which  is  necessary 
with  this  type  of  stoker,  the  general  form  and  construction  of  which  is  shown  in 
Fig.  82.  ' "  Immediately  beneath  the  coal  hopper,  and  communicating  with  it,  is 


FIG.  81.    ARRANGEMENT  OF  AMERICAN  STOKERS  AND  STURTEVANT  FAN  AT  CRANE  & 
BREED  MANUFACTURING  COMPANY,  CINCINNATI,  OHIO. 

the  conveyor,  this  in  turn  communicating  with  the  magazine  in  direct  line  with 
it.  A  screw  conveyor  or  worm  is  located  in  the  conveyor  chamber,  and  extends 
nearly  the  entire  length  of  the  magazine.  Immediately  beneath  the  conveyor 
chamber  is  located  the  wind-box,  having  an  opening  beneath  the  hopper.  At 
this  point  is  connected  the  piping  for  air  blast.  The  other  end  of  the  wind-box 


[The  American  Stoker.     Catalogue,  16  pp.     American  Stoker  Co.,  Dayton,  Ohio. 


334 


MECHANICAL    DRAFT. 


opens  into  the  air  space  between  the  magazine  and  outer  casing  or  envelope. 
The  upper  edge  of  this  magazine  is  surrounded  by  tuyeres  or  air  blocks,  these 
being  provided  with  openings  for  the  discharge 
of  the  air  blast. 


FIG.  82. 
AMERICAN  UNDER-FEED  STOKER 


"The  space  on  each  side  of  the  stoker,  between  the  tuyere  blocks  and  the 
side  walls  of  the  furnace,  is  occupied  by  dead  plates  or  air-tight  grates.  The 
coal  is  fed  into  the  hopper,  carried  by  the  conveyor  into  the  magazine,  and  is 
there  forced  upward,  '  overflowing '  on  both  sides,  and  spreading  upon  the  dead 
grates  the  entire  width  of  the  furnace  [as  shown  in  Fig.  83].  The  entire  mass 
of  coal  above  the  tuyeres  and  all  of  that 
upon  the  dead  grates  is  ignited,  carrying 
a  bed  of  burning  coke  from  14  to  18 
inches  in  depth. 

"  We  use  the  volume  blower  for  air, 
and  actuate  this  either  by  a  small  engine 
or  a  convenient  line  shaft.  The  air  is 
delivered  in  the  approximate  proportions 
of  150  cubic  feet  of  air  to  each  pound 
of  coal  fed,  and  at  a  pressure  ranging 
from  y2  ounce  to  i  ounce  at  the  tuyeres. 
This  pressure  is  only  such  as  to  admit  of 
the  thorough  mixing  of  the  air  with  the 
coal,  and  is  in  no  sense  of  the  word  a 
blast.  A  wind-gate,  controlled  by  a 
lever,  enables  the  operator  to  regulate 

the  supply  of  air  to  suit  the  amount  of    FlG<  ^    CROSS  SECTION>  SHOWING  FURNACE 
coal   fed.     Being   thus    independent   of  IN  OPERATION. 


MECHANICAL    DRAFT.  335 

natural  draft,  and  the  supply  of  coal  under  complete  control,  the  fire  can  be 
forced  at  a  moment's  notice,  and  can  be  as  quickly  reduced. 

1  "As  the  result  of  a  long  series  of  chemical  analyses  of  the  chimney  gases, 
collected  under  precisely  similar  conditions,  both  when  fired  by  hand  and  by 
stoker,  it  is  shown  that  the  amount  of  air  required  per  pound  of  coal  when 
burned  with  this  stoker,  is  from  20  to  55  per  cent  less  than  that  which  would 
have  been  used  in  the  common  hand-fired  practice.  This  effects  a  two-fold 
economy  in  decreasing  the  volume  of  heated  gases  'passing  up  the  chimney  and 
likewise  decreasing  the  velocity  of  the  gases  as  they  pass  over  the  heating  sur- 
faces, thus  allowing  more  time  for  the  absorption  of  heat  by  the  boiler  surfaces. 
Therefore,  the  increase  of  economy  must  somewhat  exceed  the  product  result- 
ing from  multiplying  the  actual  percentage  of  decrease  of  air  by  the  correspond- 
ing chimney  losses. 

"  This  economic  use  of  air  is  due  to  the  method  of  operation  peculiar  to  this 
stoker,  in  that  it  carries  a  bed  of  coal  of  unusual  depth,  the  air  being  supplied 
from  underneath,  the  volume  of  which  being  under  perfect  control,  and  the 
continuous  feeding  action  completely  overcoming  the  natural  tendency  of  holes 
burning  through  the  fire." 

Holyoke  Street  Railway  Company,  Holyoke,  Mass.  —  The  general  arrange- 
ment of  this  plant  is  very  clearly  shown  in  Fig.  84.  The  boilers  are  at  present 
three  in  number,  of  the  Babcock  &  Wilcox  water  tube  type,  of  200  horse-power 
nominal  rating  each,  set  in  a  single  battery,  and  operating  under  125  pounds 
steam  pressure.  Eventually  another  battery  of  three  will  be  in  a  corresponding 
position  on  the  other  side  of  the  fans.  The  gases,  after  leaving  the  boilers, 
pass  through  a  Green  economizer,  and  thence  to  the  brick  chamber  beneath  the 
two  Sturtevant  steam  fans.  A  damper  in  the  connection  between  the  fans,  and 
another  in  the  outlet  connection  above,  make  it  possible  to  pass  the  gases 
through  either  or  both  of  the  fans.  By  a  special  arrangement  of  automatic 
control,  the  speed  is  so  regulated  as  to  maintain  a  practically  constant  steam 
pressure. 

Although  the  demand  upon  the  fans  has  as  yet  been  comparatively  light, 
because  only  half  of  the  proposed  boilers  have  been  installed,  nevertheless  the 
adaptability  of  mechanical  draft  to  extremely  variable  conditions  of  electric  rail- 
way work  has  been  emphatically  proven.  It  is  stated  that  they  2 "  are  having 


1  The  American  Mechanical  Stoker.     C.  II.  Bierbaum,  The  Electrical  Engineer.     New  York, 
November  18,  1896. 

2  Holyoke  Street  Railway  Company,  Holyoke,  Mass.     Letter  of  March  26,  1896,  to  B.  F. 
Sturtevant  Co. 


336 


MECHANICAL    DRAFT. 


good   results  as   compared  with   statements   from   other   stations.     The   forced 
draft  works  nicely,  and  we  think  that  it  'fills  the  bill'  thus  far. 

In  February  we  ran  a  daily  average  of          .          .          19.14  hours. 

Coal  consumed  per  hour 524  pounds. 

Electrical  horse-power  per  hour    ....  243  horse-power. 
Coal  consumed  per  electrical  horse-power  per  hour,  2. 1 9  pounds. 


FIG 


INDUCED-DRAFT  PLANT  WITH  STURTEVANT  FANS  AT  HOLYOKE  STREET  RAILWAY 
COMPANY,  HOLYOKE,  MASS. 


1  "  We  know  that  it  has  more  than  fulfilled  our  expectations,  and  that  it  is 
more  economical  than  represented  by  you  to  be.      It  is  extremely  convenient  to 


'  Holyoke  Street  Railway  Company,  Holyoke,  Mass.     Letter  of  August  5,  1897,  to  B.  F. 
iiturtevant  Co. 


MECHANICAL    DRAFT. 


337 


have  precisely  the  draft  desired  at  any  time,  and  in  all  kinds  of  weather.  The 
repairs  have  amounted  to  very  little  indeed,  and  the  whole  of  it,  in  a  nutshell,  is 
that  we  are  much  pleased  with  it,  and  would  not  exchange  it  for  a  huge  chimney 
for  any  consideration." 

From  the  records  for  the  month  of  August,  1897,  the  average  results  in  Table 
No.  133  have  been  determined.  The  plant  is  usually  started  up  about  5.15  a.  m. 
and  stopped  at  12.15  a.m.,  and  all  three  boilers  are  seldom  used. 

Table  No.  133.  —  Average  Results  of  Record  of  Steam  Plant  with  Sturtevant  Induced 
Draft  at  Holyoke  Street  Railway  Company,  Holyoke,  Mass. 


ITEMS. 

Average  for  Month 
of  Aug.,  1897. 

Total  time  in  operation     .         .         .         . 

hours,                569 

Total  coal  consumed          

pounds, 

415.591 

Total  ash           

.      pounds, 

3°>897 

Total  combustible  consumed    .... 

.     pounds, 

384,694 

Average  coal  consumed  per  hour 

.     pounds, 

730-4 

Total  water  pumped  into  boilers 

pounds, 

4,150,672.4 

Average  water  pumped  into  boilers  per  hour    . 

pounds, 

7.294-7 

Average  water  pumped  into  boilers  per  pound  of 

coal  consumed    .     pounds, 

9-99 

Average  temperature  of  water  entering  economizer          .         .    degrees  Fahr., 

208 

Average  temperature  of  water  entering  boiler 

.    degrees  Fahr., 

242.7 

Total  electrical  output      

Watts, 

154,150,000 

Average  electrical  output  per  hour  . 

.        Watts, 

270,900 

Equivalent  electrical  horse-power  per  hour 

363-1 

Average  coal  per  electrical  horse-power  per  hour 

pounds, 

2.12 

Farr  Alpaca  Company,  Holyoke,  Mass.  —  This  plant,  which  is  located  at  the 
No.  i  mill,  presents  a  somewhat  novel  arrangement  of  fans,  whereby  a  relay  is 
provided  and  the  floor  area  occupied  is  reduced  to  a  minimum.  Each  fan  is  of 
special  form,  having  a  wheel  7  feet  in  diameter,  and  being  driven  by  a  direct- 
connected  6x5  double  upright  engine.  By  means  of  an  arrangement  of 
dampers,  the  gases  may  be  caused  to  pass  through  the  economizer,  and  thence  to 
either  one  or  both  of  the  fans,  whence  they  are  discharged  through  the  short, 
vertical  stack,  or  in  case  the  economizer  is  out  of  repair,  or  it  is  not  desired  to 
use  it,  the  gases  may  be  by-passed  and  enter  the  fans  direct.  The  general 
arrangement  of  the  complete  plant  is  illustrated  in  Fig.  85. 

As  stated,1  "it  was  decided  to  use  mechanical  draft  on  this  plant  principally 
because  there  was  very  poor  foundation  for  a  chimney,  and  very  little  room  to 


Samuel  M.  Green,  Consulting  Engineer.     Letter  of  Oct.  n,  1897,  to  B.  F.  Sturtevant  Co. 


338 


MECHANICAL    DRAFT. 


lU-LLLLLLLLLLLf 


MECHANICAL    DRAFT. 


339 


place  one.  The  fans  were  put  in  of  ample  capacity  to  handle  four  Manning 
boilers,  of  which  there  are  two  now  in  position  ;  one  more  is  about  to  be  in- 
stalled. Since  the  fans  were  placed  in  the  boiler  room  and  gotten  into  working 
order,  they  have  been  very  satisfactory.  The  boilers  are  of  the  Manning  up- 


2O  40  60  80  1OO  12O          140  l6o  l8o  20O          22O          240  2OO 

Revolutions  per  Minute 

FIG.  86.    RESULTS  OF  TESTS  OF  INDUCED-DRAFT  PLANT  AT  FARR  ALPACA  COMPANY, 
HOLYOKE,  MASS. 

right  type,  each  boiler  containing  180  2*^ -inch  tubes,  15  feet  long;  fire  box  6 
feet  in  diameter.  The  area  of  the  grate  is  28.27  sq.  ft.;  the  heating  surface  in 
each  boiler  is  1,823  scl-  ft-  The  economizer  contains  192  tubes  4^-inches  dia- 
meter, 4  pipes  wide,  and  48  sections.  The  square  feet  of  heating  surface  is  2,304. 


34° 


MECHANICAL    DRAFT. 


"I  have  made  a  series  of  tests  upon  the  fan  and  fan  engines  to  determine 
the  power  of  the  draft,  the  horse-power  consumed  by  the  fan  engines  and  the 
steam  used  per  horse-power/' 

The  results  thus  obtained  furnish  an  interesting  commentary  upon  the  rela- 
tions between  fan  speed,  volume  moved,  pressure  created  and  horse-power 
required,  which  relations 'have  already  been  discussed  at  length  in  a  preceding 
chapter.  In  Fig.  86  these  results  are  graphically  presented  in  such  a  manner  as 
to  make  evident  the  conditions  under  which  the  various  changes  took  place.  No 


42.5.    Revs. 


61.5      Revs. 


106.5    Revs. 


131.5     Revs. 


162     Revs. 


200      Revs. 


225       Revs.  256      Revs. 

FIG.  87.     INDICATOR  CARDS  FROM  STURTEVANT  ENGINE  ON  INDUCED-DRAFT  PLANT  AT 
FARR  ALPACA  COMPANY,  HOLYOKE,  MASS- 

record  of  temperatures  was  taken,  and  consequently  accurate  comparison  can- 
not be  made.  The  air  volume  was  measured  at  the  ashpit  doors  by  means  of 
an  anemometer.  The  curve  displays  the  relative  volumes  admitted,  but  obvi- 
ously does  not  indicate  the  amount  which  passed  through  the  fans.  The  rapid 
increase  in  the  power  required  when  the  speed  and  resultant  pressure  increase 
is  clearly  shown,  both  by  the  curve  upon  Fig.  86  and  even  more  clearly  by 
the  accompanying  reproductions  in  Fig.  87,  of  cards  taken  from  one  of  the 
engines  at  the  progressive  speeds  given  beneath  the  respective  cards.  Up  to 


MECHANICAL   DRAFT.  341 

a  certain  speed,  the  natural  draft  of  the  short  stack  is  equal  to,  or  actually  ex- 
ceeds, that  created  by  the  operation  of  the  fans ;  as  a  consequence,  the  work 
done  at  the  lower  speeds  is  very  slight  indeed.  But  when  the  draft  produced 
by  the  fans  exceeds  that  which  the  stack  is  capable  of  creating,  the  additional 
work  is  thrown  upon  the  fans,  and  the  power  increases  practically  as  the  cube  of 
the  number  of  revolutions. 


FIG. 


ARRANGEMENT  OF   STURTEVANT  FAN  FOR   MECHANICAL  DRAFT  AT  POPE  TUBE 
COMPANY,  HARTFORD,  CONN. 


The  Pope  Tube  Company,  Hartford,  Conn.  —  This  boiler  plant,  which  is  a 
model  in  its  way,  is  thus  briefly  described  :  '  "  At  the  present  time  there  are  in- 
stalled four  horizontal  tubular  boilers,  78  inches  in  diameter  by  18  feet  in  length, 
each  containing  151  3-inch  tubes,  18  feet  long,  and  designed  to  work  under  a 
pressure  of  135  pounds.  The  grates  are  of  the  Coxe  travelling  type,  6  feet  wide, 
and  move  at  a  maximum  rate  of  12  feet  per  hour.  The  coal  used  is  No.  2 
buckwheat  anthracite. 


The  Iron  Age,  New  York,  March  4,  1897. 


342 


MECHANICAL    DRAFT. 


"  The  draft  is  forced  by  a  Sturtevant  66-inch  fan  direct-connected  to  a  single- 
cylinder  5^  x  8^ -inch  engine,  which  also  drives  the  grates.  The  speed  of  the 
fan  is  regulated  automatically  by  the  steam  pressure  acting  through  a  Locke 
damper  regulator,  so  that  the  rate  of  grate  travel  and  blast  pressure  keeps  the 
steam  pressure  constant,  irrespective  of  the  demand  for  steam." 


FIG.  89.     STURTEVANT  FAN  FOR  MECHANICAL  DRAFT  AT  POPE  TUBE  COMPANY, 
HARTFORD,  CONN. 

In  Fig.  88  is  presented  a  front  view  of  one  of  the  boilers,  to  the  left  of  which 
and  in  the  rear  is  shown  the  Sturtevant  fan  with  direct-connected  engine.  A 
full  view  of  the  inlet  side  of  this  fan  is  shown  in  Fig.  89.  The  air  is  discharged 
upward  from  the  outlet  of  the  fan,  enters  a  horizontal  main  above  the  boilers 
and  thence  is  delivered  to  individual  rectangular  pipes  which  connect  with  the 
ashpits.  One  of  these  pipes  is  shown  in  each  of  the  illustrations.  The  cham- 
bered arrangement  of  the  ashpit,  within  which  various  pressures  are  maintained, 
has  already  been  described  in  connection  with  the  report  upon  the  Deringer 
Colliery.  A  pulley  upon  the  extended  shaft  [see  Fig.  89]  serves  as  a  means  of 
driving  the  travelling  grate. 


MECHANICAL    DRAFT.  343 

S.  S.  St.  Louis  and  St.  Paul.  —  These  twin  ships  of  the  American  Line,  Inter- 
national Navigation  Company,  are  each  equipped  with  eight  special  Sturtevant 
steel-plate  steam  fans  for  the  production  of  the  requisite  draft.  The  ships  are 
twin-screwed,  each  being  provided  with  2  six-cylinder  quadruple  expansion 
engines,  having  cylinders  as  follows :  two  high  pressure,  28^  inches  diameter, 
one  first  intermediate,  55  inches  diameter,  one  second  intermediate,  77  inches 
diameter,  and  two  low  pressure,  77  inches  in  diameter.  '  "  Steam  for  the  main 
engines  is  supplied  by  ten  boilers  of  the  Scotch  type,  six  of  which  are  double- 
ended  and  four  single-ended.  They  are  all  about  15^  feet  in  diameter;  the 
double-enders  being  20  feet  long,  and  the  single  about  half  that  length.  Each 
boiler  has  four  furnaces,  eight  of  course  in  the  double-enders,  making  64  fur- 
naces in  all,  each  with  18  square  feet  of  grate,  giving  a  total  grate  surface  of 
1,144  square  feet.  The  total  heating  surface  is  40,320  square  feet,  giving  a  ratio 
of  a  little  over  36.  Imagine  a  surface  200  feet  square  covered  with  boiling  water 
with  a  fire  35  square  feet  below  it  forced  to  a  white  heat  by  a  hot  blast,  and  burn- 
ing 300  tons  of  coal  a  day,  and  you  have  an  idea  of  the  magnitude  of  the  steam- 
generating  plant  of  one  of  these  magnificent  vessels.  The  boilers  are  arranged 
in  two  groups,  or  batteries,  each  battery  in  a  water-tight  compartment.  They  set 
fore  and  aft,  or  lengthwise  of  the  ship,  three  of  the  boilers  side  by  side  and  two 
of  the  small  ones  facing  them,  in  each  compartment.  .  .  .  The  Howden  system 
of  forced  draft  is  used.  Each  stokehole,  of  which  there  are  four,  is  supplied  with 
two  So-inch  Sturtevant  fans,  each  driven  by  two-cylinder  8x5^-inch  engines 
directly  connected  to  the  fan  shaft.  [See  Fig.  91.]  These  fans  draw  the  air 
from  the  top  of  the  stokeholes,  force  it  through  the  tubes  in  the  uptakes  of  the 
boilers,  and  pass  it  to  the  casing  or  chamber,  which  will  be  seen  protruding  from 
the  boiler  front." 

The  general  arrangement  of  these  tubes  in  the  uptakes  is  very  clearly  sho"wn 
in  the  accompanying  Fig.  90,  which  is  from  a  photograph  of  one  of  the  double- 
ended  boilers  for  the  St.  Paul.  The  fans  are  of  special  construction  and  rigidly 
attached  to  the  water-tight  bulkheads. 

"Directly  above  each  furnace  door,  a  small  crank  will  be  seen.  This  con- 
trols the  admission  of  air  to  a  chamber  in  the  door  of  the  furnace,  whence  it 
issues  through  the  perforated  plate  and  is  delivered  over  the  fire.  At  the  side 
of  each  furnace  is  another  handle  which  controls  the  admission  of  air  below 
the  grate.  .  .  .  The  fire  tubes  are  provided  with  spiral  deflectors  [retarders] 
to  retard  the  passage  of  gases  and  keep  them  impinging  against  the  heating 
surface." 


Steam  Plant  of  the  St.  Paul  and  St.  Louis.  Power,  New  York  and  Chicago.    February,  1896. 


344 


MECHANICAL    DRAFT. 


MECHANICAL   DRAFT. 


345 


The  chief  advantages  claimed  for  this  system  are  '"(i)  increase  of  power, 
(2)  economy  in  fuel,  (3)  reduced  wear  and  tear  of  boilers,  (4)  coolness  in  fire- 
holds,  (5)  reduced  size  and  weight  of  boilers  for  a  given  power,  (6)  simplicity. 

"  First.  The  power  from  a  boiler  of  a  given  size  may  be  increased  by  the  use 
of  this  system  with  safety  and  comparative  economy  40  per  cent  over  that 
obtainable  by  natural  draft.  Mr.  Howden  claims  in  special  cases  100  per  cent 


FIG.  91.     STURTEVANT   SPECIAL  STEEL-PLATE  STEAM  FAN  FOR  FORCED  DRAFT  ON 
S.  S.  ST.  Louis  AND  ST.  PAUL. 

increase,  but  it  is,  however,  only  in  special  cases  that  such  a  great  increase  is 
necessary  or  desirable.  In  ordinary  merchant  steamers  from  40  to  60  per  cent 
increase  of  power  is  obtained,  or  from  1 6  to  19  indicated  horse-power  per  square 
foot  of  fire  grate.  Such  powers  are  obtained  with  a  very  high  economy  in  fuel. 


i  The  Howden  System  of  Hot  Draft.     Dry  Dock  Engine  Works,  Detroit,  Mich.     Novem- 
ber, 1896. 


346  MECHANICAL    DRAFT. 

"  Second.  The  high  economy  in  fuel  obtained  by  this  system  of  combustion 
is  one  of  its  most  valuable  features,  and  has  contributed  largely  to  its  adoption. 

"  The  greatly  superior  economy  of  the  Howden  draft  system,  compared  with 
the  best  examples  of  natural  draft,  has  been  completely  proved  by  the  British 
India  Steam  Navigation  Company,  who  built  two  large  steamers  for  the  purpose 
of  comparing  results.  These  steamers  had  hulls  and  engines  exactly  alike,  .the 
only  difference  being  in  the  boilers  :  one  of  the  steamers,  Vadala,  having  two 
double-ended  boilers  with  eight  furnaces  worked  by  natural  draft;  the  other 
vessel,  the  Virawa,  having  two  single-ended  boilers  with  four  furnaces  worked 
by  the  Howden  system. 

"The  steamers  were  worked  in  the  same  trade,  and  all  particulars  of  consump- 
tion, speed,  etc.,  carefully  noted  by  the  company  over  a  period  of  three  years. 
Every  precaution  was  taken  to  arrive  at  a  correct  comparison  by  changing  en- 
gineers from  one  ship  to  the  other,  etc. 

"  The  result  of  over  three  years'  working  was  found  to  be  that,  while  the 
Virawa,  with  the  Howden  hot  draft,  had  a  fully  higher  average  speed,  her  con- 
sumption of  fuel  was  fully  20  per  cent  less  than  the  Vadala  with  natural  draft 
boilers.  The  effect,  however,  of  the  improvements  on  our  system,  which  we 
have  introduced  during  the  last  few  years,  has  been  to  greatly  increase  this 
economy.  The  economy  of  the  Howden  system  is  supposed  by  many  engineers 
to  be  measured  merely  by  the  units  of  heat  utilized  by  heating  the  air  of  com- 
bustion from  the  waste  gases.  This  view  entirely  overlooks  the  value  of  several 
important  collateral  effects  which  contribute  largely  to  the  economy  of  the  system. 
These  will  be  understood  better  by  the  following  explanation  :  — 

"  By  whatever  amount  the  air  of  combustion  is  increased  in  temperature  by 
the  waste  gases,  the  average  temperature  of  the  furnaces  is  practically  raised  to 
the  same  extent. 

"  From  this  increased  furnace  temperature  several  distinct  economical  effects 
arise :  (i)  By  increasing  the  evaporative  efficiency  of  the  heating  surface. 
(2)  The  gases  from  the  burning  fuel  combine  more  readily  with  the 
oxygen  of  the  air  of  combustion  as  the  temperature  of  the  fire  increases,  and 
consequently  less  air  is  required  for  combustion  per  unit  of  coal  at  the  higher 
temperature.  (3)  This  reduction  in  quantity  of  air  required  per  unit  of  coal 
consumed  has  also  an  important  economical  effect.  The  furnace  temperature  is 
increased  by  having  less  air  to  heat  to  the  furnace  temperature  ;  and  less  heat 
is  likewise  carried  off  by  the  chimney  gases.  Further,  the  volume  of  gases 
passing  through  the  boiler  being  less  in  a  given  time,  its  velocity  is  less,  and 
thus  the  hot  gases  are  longer  in  contact  with  the  evaporating  surface,  and  impart 
a  greater  proportion  of  heat  to  the  water. 


MECHANICAL    DRAFT. 


347 


"Third.  The  reduction  in  wear  and  tear  of  boilers  arising  from  the  use  of  our 
system  is  now  also  well  established.  .  .  .  The  following  facts  confirm  the 
durability  of  boilers  worked  with  the  Howden  system  of  draft :  The  first  boiler 
(marine  type),  made  thirteen  years  ago  for  experimental  purposes,  has  since 
supplied  steam  continuously  for  driving  the  Howden  works.  .  .  .  The  first 
boiler  fitted  in  a  steamship  with  our  system  was  that  of  the  New  York  City. 
This  steamer  was  sold  not  long  since  with  the  boiler  in  excellent  order,  with  the 
original  tubes,  furnaces  and  combustion  chambers. 

Table  No.  134.  —  Results  from  Log  of  Round-Trip  Voyage  of  S.  S.  St.  Louis,  Oper- 
ating under  Forced  Draft  with  Howden  System  and  Sturtevant  Fans. 


ITEMS. 

Voyage  No.  33. 

Voyage  No.  34. 

Direction  bound    ..... 

East. 

West. 

Date  of  departure          .... 

.  !     Sept.  i,  1897. 

Sept.  ii,  1897. 

Date  of  arrival      

: 

Sept.  8,  1897. 

Sept.  17,  1897. 

Time  of  passage,  dock  to  dock     . 

days,  hrs.,  mins., 

6:14:29 

6:11:15 

Time  of  passage,  sea    .... 

days,  hrs.,  mins., 

6:  10:  14 

6:7:1 

Average  steam  pressure 

pounds, 

198 

198 

Average  pressure,  fan  discharge    . 

inches  of  water, 

2-5 

2-5 

Average  pressure  in  air  reservoirs 

inches  of  water, 

2.0 

2.0 

Average  pressure  in  ashpits  . 

i-5 

!-5 

Average  indicated  horse-power 

17-863 

20,768 

Knots  per  hour      

19.95 

20.22 

Coal  per  I.  H.  P  

pounds, 

!-59 

i-59 

Coal  per  hour  per  square  foot  of  grate 

pounds, 

24.88 

28.88 

Indicated  horse-power  per  square  foot  of  grate     . 

15.61 

18.15 

Indicated  horse-power  per  square  foot  of 

heating  surface    . 

0-443 

o-5i5 

Temperature  in  air  reservoirs 

.    degrees  Fahr., 

263 

263 

Temperature  in  funnel  .... 

.    degrees  Fahr., 

564 

576 

Temperature  of  atmosphere 

.    degrees  Fahr., 

61 

61 

Temperature  of  fire  room     . 

.    degrees  Fahr., 

105 

112 

"  Fourth.  The  greater  coolness  of  the  fire-holds  in  steamers  using  our  hot 
draft  is  caused  by  the  absence  of  radiation  of  the  heat  of  the  furnaces  and  ash- 
pits into  the  fire-holds. 

"  Fifth.  That  the  space  in  steamships  occupied  by  the  boilers  worked  with 
our  system  is  -very  much  less  than  in  steamers  having  natural  draft  boilers  of 
equal  power  is  evident  from  the  much  greater  power  obtained  with  our  system 
from  boilers  of  a  given  size.  The  weight  of  the  boilers  is  proportionately  less 
with  our  system. 


348  MECHANICAL    DRAFT. 

"  Sixth.  For  simplicity  this  system  above  all  others  recommends  itself.  It 
is  a  usual  thing,  when  fitting  our  system  in  steamers  where  two  double-ended 
boilers  with  two  fire-holds  have  been  used  for  natural  draft  working,  to  use  two 
single-ended  boilers,  with  half  the  number  of  furnaces,  and  one  fire-hold  only, 
thus  saving  a  large  space  in  the  ship,  and  also  reducing  the  number  of  firemen 
and  trimmers  to  about  one-half  for  the  same  power." 

The  general  results  of  the  operation  of  this  system  of  mechanical  draft  are 
exemplified  in  Table  No.  134,  which  is  compiled  from  the  logs  of  a  round-trip 
voyage  of  the  S.  S.  St.  Louis.  The  coal  per  hour  per  indicated  horse-power  as 
therein  given,  includes  that  required  for  the  production  of  all  steam  used  at  sea 
for  the  auxiliary  engines  and  other  purposes. 

S.  S.  Kensington  and  Southwark.  —  These  two  vessels  of  the  Red  Star  Line, 
International  Navigation  Company,  are  practically  identical  in  dimensions  and 
design,  and  are  each  equipped  with  five  special  Sturtevant  fans,  in  connection 
with  the  "Ellis  &  Eaves"  system  of  heat  abstraction  and  induced  draft.  The 
general  features  of  this  system  have  already  been  presented  at  considerable 
length,  in  connection  with  the  description  of  the  plant  at  the  American  Line, 
Pier  14.  A  description  of  one  of  these  vessels  and  its  equipment  is  practically 
a  description  of  the  other,  therefore,  the  following  is  applicable  to  both  :  — 

1  "  The  S.  S.  Kensington  of  the  International  Navigation  Company  is  a  twin- 
screw  vessel  of  8,670  tons,  and  her  principal  dimensions  are  —  length,  494  feet 
by  57  feet  beam.  She  has  two  sets  of  quadruple  expansion  engines,  the  diame- 
ter of  the  cylinders  being  25^  inches,  37^  inches,  52^  inches,  and  74 
inches  by  54-inch  stroke.  The  working  pressure  is  200  pounds  per  square  inch. 
The  steam  is  generated  in  three  Scotch  boilers,  two  of  which  are  double-ended 
and  one  single-ended.  The  double-ended  are  15  feet  9  inches  diameter  by  21 
feet  5  inches  long,  while  the  single-ended  is  of  the  same  diameter  but  1 1  feet  3 
inches  long.  There  are  four  furnaces  at  each  end,  making  20  in  all,  each  with 
a  Purvis  flue.  These  are  3  feet  4  inches  mean  diameter.  The  length  of  the 
grate  is  5  feet  9  inches,  and  the  total  grate  area  383  square  feet.  The  total 
heating  surface  is  12,176  square  feet.  The  boilers  are  fitted  with  Serve  tubes 
3^  inches  in  diameter,  with  i^-inch  space  between.  The  '  Ellis  &  Eaves ' 
system  of  induced  draft  is  fitted,  and  the  boilers  are  in  one  compartment  and 
exhaust  into  one  chimney  stack,  which  is  84  feet  high  from  the  grate  level  and 
elliptical  in  plan,  14  feet  by  9  feet.  The  fans,  of  which  there  are  five,  are 
situated  at  the  base  of  the  funnel  and  are  driven  direct  by  Sturtevant  engines. 
They  are  7  feet  6  inches  in  diameter.  These  fans  induce  a  draft  through  the 


Chief  Engineer  Snowdon,  of  S.  S.  Kensington.     Letter  to  B.  F.  Sturtevant  Company. 


MECHANICAL    DRAFT. 


349 


furnaces,  the  air  having  previously  been  heated  by  passing  through  tubes  placed 
in  a  casing  over  the  boilers,  in  the  way  of  the  waste  gases  from  the  furnaces. 
The  waste  gases,  after  leaving  the  furnace,  play  around  these  tubes  forming  the 
air  inlet,  and  subsequently  pass  through  the  fans  into  the  funnel.  Each  furnace 
has  valves  above  and  below  the  grate  for  regulating  the  supply  of  hot  air,  and 
the  furnace  doors  have  dampers  connected  to  them  so  that  upon  opening  the 
door  the  draft  to  that  fire  is  minimized.  The  fans  are  so  arranged  that  one 
may  draw  from  eight  fires  if  necessary.  At  the  trials  on  the  measured  mile, 
the  mean  draught  was  21  feet  8  inches."  The  gen- 


eral arrangement  of  the 
heat  abstractors  is  very 
accompanying  cross  sec- 


main  boilers,  fans 
clearly  indicated  in 
tion,  Fig.  92. 


and 
the 


FIG.  92.     ARRANGEMENT  OF  MAIN  BOILERS,  FANS  AND  HEAT  ABSTRACTORS  ON 
S.  S.  KENSINGTON. 


35° 


MECHANICAL    DRAFT. 


Four  fans  are  grouped  beneath  the  funnel,  as  shown  in  Fig.  93,  each  fan 
being  continuously  driven  at  high  speed  by  a  double-cylindered  Sturtevant  en- 
gine. The  hot  gases  from  the  abstractors  pass  to  the  inlet  connections  between 
the  fans,  and  thence  through  the  fans  to  the  funnel  above.  A  fifth  fan  of  the 
same  design  is  applied  to  the  donkey  boiler. 


FIG.  93.     STURTEVANT  STEAM  FANS  FOR  PRODUCTION  OF  INDUCED  DRAFT  IN  CONNECTION 
WITH  ELLIS  &  EAVES  SYSTEM  ON  S.  S.  KENSINGTON. 

Table  No.  135.  —  Sample  Set  of  Data  for  One  Day  on  S.  S.  Kensington,  under  Ordi- 
nary Working  Conditions  of  Boilers,  Using  Ellis  &  Eaves  Induced 
Draft  with  Sturtevant  Fans. 


Velocity  of  air  entering  furnace        .         .         .         .  feet  per  minute, 

Velocity  of  air  entering  heating  tubes  ....  feet  per  minute, 
Temperature  of  air  entering  heating  tubes  ....  degrees  Fahr., 
Temperature  of  air  entering  fires  after  passing  through  heat-  ^  ,  p  , 


ing  tubes, 

Temperature  of  waste  gases     .         .         .         . 
Vacuum  at  fan            .   .      »    "     . 
Vacuum  below  grate          . 
Vacuum  above  grate          .         .         .         .         . 
Average  revolutions  of  Sturtevant  fans    . 
Coal  burned  per  hour  per  square  foot  of  grate 
Coal  per  I.  H.  P.  per  hour 
Indicated  horse-power 


.    degrees  Fahr., 

inches  of  water, 

inches  of  water, 

inches  of  water, 

revolutions  per  minute, 

pounds, 


3-716 
895 
140 

295 
446 

3-5 
0.687 

0-75 
39 r 
29-5 
1.4 
8,000 


MECHANICAL    DRAFT. 


351 


"  Six  runs  were  made  and  the  mean  results  were  as  follows :  — 
"Steam  pressure  199.5  pounds;  air  pressure  in  stokehold,  T3F  inches ;  revo- 
lutions of  port  engine,  86.4  ;  revolutions  of  starboard  engine,  86.9  ;  indicated 
horse-power,  port  engine,  4,074;  ditto  of  starboard  engine,  4,239;  total  indi- 
cated horse-power,  8,313.  Vacuum  was  27  inches;  speed,  15.8  knots.  As  weight 
is  an  interesting  element  in  view  of  the  system  of  forced  draft,  it  should  be 
stated  that  the  indicated  horse-power  per  ton  of  engines  is  12  ;  per  ton  of 
boilers,  12.4;  and  per  ton  of  machinery,  6.  The  following  is  a  sample  set  of 
data  for  one  day  under  ordinary  v/orking  conditions  of  the  boilers  [see  Table 
No.  135]." 

From  the  log  of  a  round-trip  voyage  of  S.  S.  Kensington,  Table  No.  136 
has  been  compiled,  which  serves  to  indicate  in  a  general  way  the  conditions 
existing  and  the  results  obtained.  In  the  calculation  of  coal  consumed  per  in- 
dicated horse-power,  there  has  been  included  the  amount  used  at  sea  for  the 
auxiliaries  and  all  other  purposes  on  board  the  ship.  The  measurement  of 
power  from  steam  made  by  the  boilers  is  indicated  by  that  passing  through  the 
main  engines  only. 

Table  No.   136.  —  Results  from  Log  of  Round-Trip  Voyage  of  S.  S.  Kensington,  Oper- 
ating under  Ellis  &  Eaves  Induced  Draft  with  Sturtevant  Fans. 


ITEMS. 

Voyage  No.  30. 

Voyage  No.  31. 

Direction  bound     ..... 

East. 

West. 

Date  of  departure          .... 

.  >    June  30,  1897. 

July  24,  1897. 

Date  of  arrival      

.  i    July  it,  1897. 

August  3,  1897. 

Time  of  passage,  dock  to  dock 

days,  hrs.,  mins.,           10  :  13  :  29 

10:4:30 

Time  of  passage,  sea     .... 

days,  hrs.,  mins.,  |          10:6:  16 

9-*5:3« 

Average  steam  pressure 

pounds, 

195 

r93-S 

Average  fan  suction       .... 

inches  of  water, 

3-5 

3-o 

Average  vacuum  in  air  reservoirs 

inches  of  water, 

0.625 

0.5 

Average  vacuum  in  ashpits   . 

inches  of  water, 

0.75 

0.625 

Average  indicated  horse-power 

. 

7,841 

6,870 

Knots  per  hour      ..... 

13-74 

14.03 

Coal  per  I.  H.  P  

pounds, 

i-43 

i-59 

Coal  per  hour  per  square  foot  of  grate 

.     pounds,                  29.3 

28.66 

Indicated  horse-power  per  square  foot  of 

grate    .         .         .                  20.47 

'7-93 

Indicated  horse-power  per  square  foot  of 

heating  surface    .                   0.643 

0.564 

Temperature  of  gases  at  fan  discharge 

.  degrees  Fahr.,               427 

383 

Temperature  in  air  reservoirs 

.  degrees  Fahr., 

329 

296 

Temperature  of  atmosphere 

.  degrees  Fahr.,                62 

58 

Temperature  of  fire-room 

.   degrees  Fahr.,                83 

87 

352 


MECHANICAL    DRAFT. 


MECHANICAL    DRAFT.  353 

Washington  Garbage  Crematory,  Washington,  D.  C.  —  This  extensive  plant, 
a  view  of  which  is  presented  in  Fig.  94,  was  constructed  by  the  American 
Garbage  Cremator  Company,  and  equipped  with  a  Sturtevant  blower  of  the 
"  Monogram  "  pattern  placed  overhead.  This  blower  is  driven  by  a  double  en- 
closed upright  engine  standing  upon  the  floor  and  belted  upward,  as  shown  in 
the  illustration. 

As  described,1  "the  garbage  crematory  is  located  on  the  banks  of  the  Eastern 
branch,  directly  south  of  the  Capitol.  It  consists  of  a  series  of  furnaces  heated 
to  a  high  degree,  into  which  the  garbage  is  dumped  directly  from  the  collecting 
wagons.  The  heat  dries  the  garbage,  and  soon  incinerates  it,  making  an  ash 
that  contains  a  percentage  of  fertilizing  material  which  the  contractor  sells. 

"  The  cremation  furnaces,  for  there  are  two  of  them,  are  the  largest  reverber- 
atory  furnaces  ever  "built  in  this  country.  The  extreme  length  of  each  furnace 
is  43  feet ;  width  over  all,  1 2  feet ;  height,  1 1  feet.  The  furnaces  are  located  on 
either  side  of  a  self-supporting  steel  stack,  7  feet  in  diameter  and  120  feet  high. 
Each  furnace  is  fitted  up  with  a  Brown  patent  combustion  chamber,  which  is  a 
combination  gas  producer,  mixer  and  combustion  chamber.  In  this  chamber 
there  is  made,  and  from  it  is  forced,  what  is  known  as  a  powerfully  oxidizing 
flame,  competent  to  destroy  garbage  in  a  perfectly  sanitary  way  and  at  a  reason- 
able cost.  The  combustion  chambers  are  located  at  either  end  of  each  furnace 
farthest  from  the  stack,  and  are  connected  to  the  reverberatory  burning 
chambers,  over  and  under  which  the  oxidizing  flames  are  forced,  impinging  on 
the  upper  and  lower  sides  of  the  garbage  at  the  same  time.  The  garbage  itself 
is  deposited  on  a  horizontal  double  row  of  semi-steel  grate  bars,  which  rest  on 
the  sides  of  the  burning  chambers  on  supporting  ledges  of  fire-brick  masonry ; 
while  their  centres  are  supported  by  a  longitudinal  bridge  wall  of  the  same 
material. 

"The  flames  not  only  pass  over  and  under  the  garbage,  consuming  it  as  they 
pass,  but  a  portion  of  the  products  of  combustion  passes  back  through  an  under- 
neath chamber,  or  a  tritatory  chamber,  called  evaporating  cells,  where  all  the 
water  drained  from  the  garbage  is  deposited.  This  water  is  thus  evaporated, 
and  in  the  process  of  evaporation  forced  forward,  meeting  the  main  or  direct 
volume  of  flame  on  its  route  to  the  stack.  The  so-called  direct  flame  is  always 
competent  to  decompose  the  evaporated  water.  However,  to  make  this  doubly 
sure,  a  stack  fire  of  sufficient  intensity  is  always  in  operation,  over  which  the  en- 
tire products  of  combustion  of  both  furnaces  are  deflected,  before  reaching  an 
exit  to  the  outer  world  through  the  stack. 


[The  Evening  Star,  Washington,  D.  C.,  Sept.  12,  1896. 


354 


MECHANICAL    DRAFT. 


"  In  the  stack  is  located  a  series  of  regenerative  tubes,  through  which  air  is 
constantly  passing,  either  by  natural  or  forced  draft.  This  air,  when  drawn  by 
the  natural  draft  of  the  furnace  stack,  goes  directly  through  the  regenerative 
tubes,  thence  through  the  hollow  longitudinal  bridge  wall  supporting  the  grate 
bars,  and  thence  into  the  combustion  chambers,  thus  supplying  them  at  all  times 
with  superheated  air.  In  connection  with  this  regenerative  system,  and  coupled 
to  it,  is  a  powerful  [Sturtevant]  blower  operated  by  an  engine.  When  the 
furnaces  are  fully  charged  and  the  final  operation  in  the  destruction  of  the 
garbage  begins,  the  engine  and  blower  are  started,  thus  giving  a  more  powerful 
hot  blast  than  could  otherwise  be  obtained." 

The  general  arrangement  of  the  combustion  chamber, 
with  the  grate  bars  and  garbage  thereon,  is  clearly  shown 
in  Fig.  95.  This  represents  one-half*  of  the  plant  shown 
in  Fig.  94.  The  air  blast  from  the  blower  enters  the 
sub-chamber  through  the  pipes  which  are  shown  within 
the  stack. 


FIG.  95.     SECTION  THROUGH  COMBUSTION  CHAMBER  OF  GARBAGE  CREMATORY,  AT 
WASHINGTON,  D.  C. 

Knickerbocker  Lime  Company,  Mill  Lane  Station,  Pa.  —  Six  single  and  one 
double  kiln  constitute  this  plant.  The  apparatus  is  a  6x3^  Sturtevant  steel- 
plate  steam  fan  driven  by  direct-connected  horizontal  engine  arranged  as  shown 
in  Fig.  96.  The  air  is  discharged  vertically  upward  into  a  pipe  which,  turning  hori- 
zontally at  about  18  feet  above  the  floor,  passes  along  the  fronts  of  the  kilns. 
Each  single  kiln  is  provided  with  two  grates,  one  upon  either  side,  each  of 
about  15  feet  square  feet  area,  upon  which  about  4  tons  of  coal  are  burned  per 
day  of  24  hours.  Fig.  97  presents  a  view  of  the  front  of  the  double  kiln  and 
indicates  the  manner  of  introduction  of  air  to  the  ashpits  just  at  the  side  of  the 
ashpit  doors.  The  lime  through  which  the  hot  gases  pass  is  contained  in  a  cen- 
tral circular  chamber  of  considerable  height,  whence  it  is  drawn,  when  properly 
burned,  through  a  separate  opening. 


MECHANICAL    DRAFT. 


355 


FIG.  96.      FORCED-DRAFT  APPARATUS,  KNICKERBOCKER  LIME  Co.,  MILL  LANE  STATION,  pA 


FIG.  97.     FRONT  OF  DOUBLE  KILN,  KNICKERBOCKER  LIME  Co.,  MILL  LANE  STATION,  PA. 


356  MECHANICAL    DRAFT. 

Boston  Woven    Hose   &    Rubber    Company,    Cambridgeport,    Mass.  —  The 

horizontal  return  tubular  boilers  of  this  plant  which  were  tested  as  here  indicated, 
were  equipped  with  the  Bacon  setting,  which  is  thus  described  :  '  "  The  Bacon 
setting  is  a  new  and  useful  device,  built  on  strictly  scientific  principles,  having 
for  its  objects  the  saving  of  fuel  and  prevention  of  smoke,  or  the  complete 
combustion  of  coal.  These  objects  are  most  effectually  obtained  in  this  device. 

"  First.  The  harder  a  boiler  with  Bacon  setting  is  worked  the  better  the 
results,  both  as  regards  saving  in  fuel  and  the  completeness  of  combustion. 

"  Second.  The  admission  of  air  at  a  very  high  temperature  of  heat  being 
equal  to  that  of  the  gas,  and  when  coal  is  put  on  the  air  exceeds  that  of  the 
gas.  .  .  . 

"  Third.     Absence  of  dirt  in  back  connection. 

"  Fourth.     Cleanliness  of  tubes,  which  is  due  to  the  consumption  of  gases. 

"  Fifth.  The  increase  of  temperature  in  flue,  which  allows  shortening  of 
grate. 

"Sixth.     The  holding  of  full  pressure  of  steam  when  fires  are  banked/' 

Tests  were  made  upon  boilers  5  and  6  of  this  plant  by  Mr.  Greely  S. 
Curtis,  Jr.,  upon  Dec.  9  and  n,  1896,  2  "  to  determine  the  evaporative  efficiency 
of  Dominion  coal  when  burned  on  the  Bacon  setting  under  forced  draft.  The 
boilers  were  fired  by  the  regular  fireman,  who  was  experienced  in  the  use  of 
Dominion  coal  in  the  settings  tested;  and  they  supplied  the  plant  with  slightly 
more  than  the  customary  power,  as  the  dynamos  were  run  longer  than  in  other 
months  and  the  main  engine  and  shafting  were  driven  through  the  noon  hour." 

These  tests  having  shown  that  a  great  and  unnecessary  excess  of  air  had 
been  supplied,  another  test  was  made  on  Dec.  23.  3  "The  test  was  to  show 
the  effects  of  lessening  the  air  supply  to  the  boilers  tested  on  Dec.  9  and  n. 
Since  that  time,  various  unintentional  air  outlets,  as  well  as  several  of  the 
regular  perforations  of  the  Bacon  boiler  settings,  have  been  stopped  up,  as  was 
recommended  in  my  previous  report. 

"The  blower  was  driven  at  269.7  revolutions  per  minute,  instead  of  350  as 
previously,  and  the  forced-draft  pressure  was  reduced  from  4.9  inches  of  water 
to  3.9  inches.  .  .  .  The  general  conditions  were  similar  to  those  of  the 
previous  tests,  except  that  on  the  23d  the  boilers  had  to  supply  a  slightly  greater 
demand  for  steam.  The  following  brief  comparison  may  be  of  interest.  [See 
Table  No.  137.] 


1  The  Bacon  Setting,  Circular.     Bacon  Engineering  Company,  Boston,  Mass. 

2  Report  of  Mr.  Greely  S.  Curtis,  Jr.,  Dec.  17,  1896,  to  The  Dominion  Coal  Co.,  Boston,  Mass. 

3  Report  of  Mr.  Greely  S.  Curtis,  Jr.,  Dec.  24,  1896,  to  The  Dominion  Coal  Co.,  Boston,  Mass. 


MECHANICAL   DRAFT. 


357 


Table  No.   137.  —  Comparison  of  Results  of  Tests  of  Boilers  5  and  6  with  Bacon  Set- 
tings with  Sturtevant  Fan  at  Boston  Woven  Hose  &  Rubber  Company, 
Cambridgeport,  Mass. 


ITEMS. 

Dec.  9. 

Dec.  ii. 

Feed  water  per  pound  of  coal 

pounds, 

8.82 

8.84 

Equivalent  evaporation  per  pound  of  combus-  / 
tible  from  and  at  2120,  gross,                          } 

pounds,  : 

10.63 

11.00 

Equivalent  evaporation  per  pound  of  coal  from  j 
and    at    2120,  corrected    for   priming  and  |- 

pounds,  ! 

9-56 

9.69 

steam  to  blower  engine,  net,                            ) 

Dec.  23. 


The  general  results  of  the  second  series  of  tests  described  above  are  herewith 
presented  in  Table  No.  138. 

Table  No.   138. — Data  and    Results  of  Tests  of   Boilers  5  and  6  with'  Bacon  Settings 

with  Sturtevant  Fan  at  Boston  Woven  Hose  &  Rubber  Company, 

Cambridgeport,  Mass. 


Date  .         .  .      -  . Dec.  23,  1896. 

Duration .        hours,  H-45 

Weight  of  dry  coal  consumed            .               •    .         .         .         .          .      pounds,  |  7,876.5 

Weight  of  ashes  and  clinkers  ........      pounds,  i  631 

Weight  of  water  evaporated     .         ......         .               pounds,  80912 

Average  boiler  pressure     .        _.       ' '*         .'-...      ...        '.•".'    .     pounds,  108.52 

Average  temperature  of  feed  water degrees  Fahr.,  '.  94-7<3 

Forced  draft       .        .         .     -   .         .         .         .         .         ,         inches  of  water*  j  3.9 

Per  cent  of  moisture  in  coal      .         .         .         ....       .:."".        ."/••.!.  8.31 

Per  cent  of  moisture  in  steam  .         .         .               •  .•        .         ...         .  ' '_  ji  [•;'  i.~o 

Equivalent  evaporation,  dry  steam  per  hour,  feed  100°,  pressure  70  )             ,     !  ,.  oo_  Q 

pounds,  inclusive  of  steam  to  blower  engine,  } 

Horse-power  developed  on  A.  S.  M.  E.  basis  of  30  pounds  per  horse-power    .  229.26 

Coal  consumed  per  hour  per  square  foot  of  grate  surface        .         .      pounds,  16.41    • 
Equivalent  evaporation  per  square  foot  of  heating  surface  per  hour,  (   poun(js   i 

from  and  at  2120,  J;J 

Feed  water  per  pound  of  coal  .         ...         .         .         .  '    -  ,  ;     .     pounds,  i  10.258 

Equivalent  evaporation  per  pound  of  coal' from  and  at  2120    .         .      pounds,  \  MX^I 

Equivalent  evaporation  per  pound  of  combustible  from  and  at  212",   pounds,  '--93 

Actual  dry  steam  supplied  for  useful  work  per  pound  of  coal          .     pounds,  9-931 


The  draft  pressure  maintained  as  indicated  above  is  such  that  a  chimney 
would  be  absolutely  inadequate.  It  is  evident  that  nothing  but  mechanical 
means,  as  a  fan,  is  capable  of  producing  the  required  pressure. 


353 


MECHANICAL    DRAFT. 


Crematory,  Lee  and  Pennsylvania  Avenues,  Washington,  D.  C.  —  The  discus- 
sion of  the  relative  merits  of  cremation  and  burial  has  naturally  resulted  in  the 
perfection  of  certain  forms  of  crematory  furnaces  for  the  proper  incineration  of 


the  deceased, 
certain  features 
rapidity  of  ac- 
desired  result, 
service  in  that 
produces  the 
combu  stion. 
Brown  Incinera- 


Obviously  such  a  furnace  must  possess 
peculiar  to  itself,  among  which  are 
tion  and  perfect  accomplishment  of  the 
In  this  connection,  the  fan  is  of  special 
it  renders  the  operation  positive  and 
intensity  of  draft  desirable  in  rapid 
The  above-named  plant  consists  of  a 
tor  and  a  Sturtevant  Steel  Pressure 


FIG.  98.    LONGITUDINAL  SECTION   THROUGH  THE   BROWN  INCINERATOR  IN    CONNECTION 
WITH  WHICH  THE  STURTEVANT  FAN  IS  EMPLOYED. 

Blower,  —  the  latter,  being  behind  the  former,  does  not  appear  in  Fig.  98. 

The  general  construction  and  operation  of  the  incinerator  is  thus  described  :  — 
r"  It  requires  but  half  an  hour  in  this  incinerator  to  heat  the  retort  to  a  proper 
degree  for  receiving  the  body.  Within  two  hours  the  body  is  reduced  to  five  or 
six  ounces  of  pure  white  ashes,  and  not  one  single  particle  of  deleterious  gas 
escapes  in  any  direction  ;  and  the  cost  of  cremation  is  less  than  a  dollar  and  a 
half  for  each  time. 


i  Cremation :  Why  and  How.     The  Brown  Incinerator  Company,  Boston,  Mass. 


MECHANICAL    DRAFT.  359 

"  The  description  of  the  Brown  Incinerator  is  briefly  this:  It  is  a  structure  ten 
feet  long  by  seven  wide,  and  six  high.  It  is  built  of  fire-bricks,  surrounded  on 
four  sides  by  a  jacket  which  is  filled  with  moving  water,  while  the  top  is  covered 
with  a  layer  of  sand.  This  environment  of  water  and  sand  keeps  the  fire-bricks 
at  an  equable  temperature,  so  that  it  does  not  scale  and  crack ;  and,  moreover, 
it  so  entirely  confines  the  heat  within  the  furnace  that,  in  the  midst  of  crema- 
tion, when  in  the  retort  there  is  a  temperature  of  2,000  degrees,  one  may  lay  his 
hand  anywhere  upon  the  outside  of  the  furnace. 

"The  retort  that  receives  the  body  is  a  cylinder  eight  feet  long  by  thirty  inches 
in  diameter,  made  of  decarbonized  fire-box  steel,  which  will  endure  a  tempera- 
ture of  nearly  3,000  degrees.  In  the  forward  end  of  this  cylinder  is  a  door, 
which  closes  absolutely  air-tight.  Below  the  door  of  the  cylinder,  near  the 
ground,  enter  three  concentric  pipes,  opening  into  a  small  combustion  chamber. 
The  innermost  of  these  pipes  contains  steam,  the  second  pipe  crude  petroleum, 
while  the  third  contains  burnt  gases  which  have  been  drawn  out  from  the  com- 
bustion chamber  itself,  and  are  returned  to  it.  These  three  pipes,  emptying 
their  contents  at  the  same  point,  have  this  effect :  the  steam  gasifies  the  oil,  and 
this  gas,  in  turn,  mingles  with  the  gases  of  the  outer  pipe,  forming  a  new  gas 
of  the  highest  combustibility.  This  is  ignited  as  it  enters  the  combustion 
chamber.  Then,  in  a  high  state  of  combustion,  it  rises,  and  entirely  envelops 
the  steel  retort  in  which  the  body  is  placed,  and  circling  round  it,  passes  off  at 
the  rear  of  the  furnace  into  a  chimney.  After  combustion  has  been  in  progress 
for  half  an  hour,  the  retort  has  reached  a  heat  of  2,000  degrees.  The  body  to 
be  incinerated  is  wrapped  in  a  prepared  sheet,  and  placed  in  a  semicircular 
shallow  casket.  A  car  conveys  this  to  the  mouth  of  the  cylinder.  It  is  then 
placed  within,  and  the  door  closed.  The  intense  heat  of  the  retort  immediately 
decomposes  the  body,  liberating  its  particles  in  the  form  of  gas.  The  retort  is 
surrounded  by  a  jacket;  from  the  retort  a  pipe  leads  into  the  jacket,  and  from 
this  jacket  another  leads  through  the  walls  of  the  furnace,  and  connects  with  a 
steam-pipe,  through  which  a  rush  of  steam  creates,  by  means  of  a  siphonic  con- 
struction, a  strong  suction.  This  suction  is  imparted  first  to  the  jacket  around 
the  retort,  and  then  through  its  connecting  pipe  to  the  retort  itself.  When  the 
steam  is  forced  through  the  outer  pipe,  the  gases  rising  from  the  body  within 
the  retort  are  drawn  first  into  the  surrounding  jacket.  While  passing  through 
this  jacket,  they  are  subjected  for  some  time  to  a  very  high  degree  of  heat,  so 
that,  if  they  were  not  already  decomposed,  they  are  here  entirely  resolved  into 
their  constituent  gases,  and  absolutely  purified.  Then  they  pass  out  into  the 
steam,  and  are  conveyed  away.  In  this  way  it  is  absolutely  impossible  for  a 
particle  of  gas  to  escape  from  the  retort  into  the  surrounding  air.  A  small 


360  MECHANICAL    DRAFT. 

opening  at  the  foot  of  the  retort  permits  the  entrance  of  heated  air,  completing 
the  draft  over  the  burning  body.  This  draft  is  so  strong  that  even  if  the  door 
were  opened  at  the  expiration  of  an  hour,  —  after  which  time  the  greater  part 
of  the  gases  have  been  driven  away,  —  there  would  be  no  emission  of  odor  or 
of  gas  from  the  retort. 

"  The  rapidity  with  which  this  incinerator  works,  requiring  but  a  half  hour  for 
heating  and  but  one  hour  for  the  accomplishment  of  cremation,  the  very  small 
expense  of  the  fuel,  and  the  inexpensiveness  of  the  construction  itself,  make  it 
equally  serviceable  for  city  cemeteries,  where  much  work  is  to  be  done,  and 
for  rural  cemeteries,  where  the  matter  of  expense  is  a  great  consideration." 

The  air  blast  from  the  Sturtevant  blower  is  introduced  through  an  opening 
in  the  farther  side  of  the  incinerator,  which  as  already  stated  does  not  show  in 
Fig.  98,  because  of  the  intervening  retort. 

The  Riordon  Paper  Mills,  Limited,  Merriton,  Ont.  —  The  adaptability  of 
small  Sturtevant  fans  to  the  production  of  draft  in  place  of  a  chimney  is  very 
clearly  shown  by  the  experience  with  this  plant.  The  actual  cost  of  the  two 
fans  was  only  about  6  per  cent  of  the  probable  cost  of  the  chimney  as  specified 
below,  to  say  nothing  of  the  increased  convenience,  the  positive  character  and 
the  ability  to  burn  cheap  fuel.  The  plant  and  its  operation  are  thus  described : 
1  "  We  have  four  steam  boilers,  150  horse-power  =600  horse-power.  They  are 
worked  at  90  pounds  pressure,  and  feed  water  is  supplied  at  an  average 
temperature  of  150°  Fahr.  The  quantity  of  steam  used  varies  considerably  as 
the  various  machines  or  digesters  in  the  mill  are  started  or  stopped,  but  during 
the  greater  part  of  the  twenty-four  hours,  the  boilers  are  pushed  to  their 
utmost  capacity.  The  boilers  are  coupled  in  pairs  to  two  Sturtevant  Steel  Plate 
Exhausters,  Nos.  50  and  60,  both  running  at  800  revolutions  per  minute.  The 
draft  from  the  No.  50  is  2-ounce  pressure  and  from  the  No.  60  2^ -ounce 
pressure.  Pennsylvania  bituminous  run-of-mine  coal  is  used.  Consumption  is 
12  pounds  of  coal  per  square  foot  of  grate  surface  per  hour.  The  fans  are 
driven  by  a  water  motor,  and  require  3^  and  4  horse-power,  respectively. 
They  require  little  or  no  trouble  or  attention  ;  the  bearings  running  in  water, 
no  lubricant  is  used.  As  to  the  cost  of  building  a  stack  to  fulfil  the  same 
requirements,  it  would  be  great,  probably  not  less  than  $3,000,  but  apart  from 
the  question  of  cost,  want  of  space  would  make  such  a  structure  nearly  impos- 
sible. The  No.  50  fan  has  now  been  working  over  five  years  with  but  very  little 
repair,  and  the  No.  60  for  two  years  with  no  repairs  whatever." 


'The  Riordon  Paper  Mills,  Limited,  Merriton,  Ont.     Letter  of  February  iS,  1896,  to  B.  F. 
Sturtevant  Co. 


MECHANICAL   DRAFT. 


Boston  Duck  Company,  Bonds ville,  Mass.  —  In  the  case  which  is  herewith 
presented  in  Fig.  99,  a  Sturtevant  steel-plate  steam  fan  is  used  in  connection 
with  the  Columbia  stokers.  The  fan  delivers  the  air  into  an  underground  duct 
extending  along  the  fronts  of  the  boilers.  From  this  duct  special  pipes  conduct 
it  to  the  boiler  furnaces.  The  location  of  the  fan  and  the  substantial  and  un- 
obtrusive form  of  the  special  pipes  render  the  arrangement  extremely  convenient. 

The  general  method  of  operation  is  evident  in  the  longitudinal  section,  Fig.  100, 
but  is  rendered  still  clearer  by  the  detailed  explanation  which  here  follows :  — 


FIG.  99.    BOILER  PLANT  EQUIPPED  WITH  COLUMBIA  STOKERS  AND  STURTEVANT  FAN,  AT 
BOSTON  DUCK  COMPANY,  BONDSVILLE,  MASS. 

1  "  The  fuel  to  be  burned  is  shovelled  into  a  hopper,  the  top  of  which  is  located 
about  four  feet  above  the  floor  line ;  from  there  the  fuel  is  pushed  up  through  a 
slightly  inclined  fuel  passage  by  a  slide  or  coal  pusher  which  derives  its  motion 
from  an  oscillating  shaft  extending  across  the  front  of  the  stoker ;  the  amount 
of  travel  of  the  coal  pusher  being  regulated  by  a  feed  screw,  and  may  be  varied 
from  one  to  four  inches,  provision  being  also  made  to  stop  the  feeding  apparatus 
entirely  without  interfering  with  the  other  stokers  which  may  be  set  in  a  battery 
of  boilers  and  operated  by  the  same  main  shaft. 


i  The  Columbia  Mechanical  Stoker  and  Smokeless  Furnace.    Catalogue,  8  pp. 
Stoker  Company,  Holyoke,  Mass. 


The  Columbia 


362 


MECHANICAL    DRAFT. 


The  main  shaft  receives  its  oscillating  motion  direct  from  a  water  engine 
bolted  to  the  stoker  front.  The  coal  being  pushed  through  the  enclosed  fuel 
passage  is  delivered  on  to  the  blast  grates  which  rest  on  the  air  box,  which  is 
supplied  with  air  under  mild  pressure  from  a  fan  blower.  Owing  to  the  incline 
in  the  fuel  passage,  the  fresh  fuel  will  have  a  tendency  to  slide  along  the  fuel 
plate  directly  on  to  the  blast  grates,  and,  in  doing  so,  cause  the  bed  of  incandes- 
cent fuel,  in  course  of  combustion,  to  bulge  or  rise  up  ;  the  heat  from  the 
burning  fuel  will  slowly  liberate  the  gas  from  the  incoming  fresh  coal,  and  the 

air   forced  through  openings  in  the 
blast  grates    in  passing  up  through 


FIG.  100.     LONGITUDINAL  SECTION  THROUGH  FURNACE  SHOWING  CONSTRUCTION   OF  CO- 
LUMBIA MECHANICAL  STOKER  AND  ARRANGEMENT  FOR  INTRODUCING  AIR  SUPPLY. 

the  fresh  fuel  will  be  thoroughly  mixed  with  the  gases  liberated  before  passing 
through  the  burning  fuel  above,  resulting  in  a  bright,  clear  fire  and  the  complete 
consumption  of  all  combustible  elements  in  the  fuel.  The  incoming  fuel  (on 
account  of  being  pushed  up  the  incline),  is  in  a  compact  mass,  and  will  not 
permit  cold  air  to  pass  into  the  furnace,  as  the  whole  grates  are  covered  with 
an  even  layer  of  coal.  This  process  of  feeding  in  fresh  coal  which  raises  or  re- 
places the  ignited  fuel  is  going  on  continuously,  the  resulting  ash  and  clinkers 
being  gradually  forced  over  the  top  on  to  the  inclined  grates  for  final  combus- 
tion, air  being  supplied  through  the  openings  in  the  grates  induced  by  ordinary 
chimney  draft. 


MECHANICAL    DRAFT.  363 

"  A  drop  or  dump  grate,  provided  for  the  removing  of  clinkers  and  ashes,  is 
located  on  the  lower  end  of  the  inclined  grates,  and  operated  by  handle  bars 
extended  through  the  front  of  the  furnace.  The  whole  operation  of  removing 
the  clinkers  and  ashes  only  requires  a  few  minutes,  and  can  be  done  without 
interfering  with  the  regular  course  of  firing. 

"  To  secure  the  best  results  a  heavy  body  of  cake  should,  at  all  times,  be 
carried  in  the  front  of  the  furnace,  and  as  the  amount  of  coal  burned  is  regu- 
lated by  the  quantity  of  air  supplied,  the  carrying  of  a  heavy  fire  during  the 
time  when  little  steam  is  needed,  will  not  cause  loss  in  fuel,  as  the  air  supply  to 
the  stokers  is  regulated  by  an  improved  damper  regulator." 

The  relative  efficiency  with  which  this  plant  operated  without  and  with 
stokers  and  forced  draft  is  evidenced  in  these  statements.  The  tests  cover  a 
sufficient  period  to  render  the  results  conclusive. 

" '  The  following  is  an  evaporative  test  of  our  boilers  before  the  stokers  were 
applied  to  the  same  :  — 

Weeks  ending  — 

Oct.  3,  1886,  water  evaporated  per  pound  of  coal  from  and  at  212°,    9.9  pounds. 
"    10,       "          "  "  "  "          "  10.0        '" 

"    17,       "          "  "  "          "  10.1        '• 

«    24,       "          "  "  "  "          '•  9.9        fe 

"    31,       "          "  "  "  "          '"  <:  i  o-o       " 

Average 9.98  pounds. 

"  The  following  figures  represent  a  test  made  under  conditions  as  above,  after 
the  stokers  were  applied  :  — 

Weeks  ending  — 
Sept.  4,  1897,  water  evaporated  per  pound  of  coal  from  and  at  212°,  11.7  pounds. 

"     18,     "         "  "  "  "         "  "  1 1. 6       " 

Oct.    16,     "          "  "  "  "         "  "  "          11.4       " 

u       30?        <«  u  u  «  «  II-4  « 

Nov.  13,     "         "  ii. 6       " 

Average 11.5  4  pounds. 

"These  figures  do  not  represent  selected  cases,  as  we  did  not  make  them 
continuous  every  week,  but  made  them  occasionally.  The  latter  tests  we  made 
with  coal  which  I  consider  to  be  8  to  10  per  cent  inferior  in  quality,  and  which 
cost  us  7  per  cent  less  than  the  coal  used  in  the  hand-fired  tests.  These  stokers 
have  been  in  operation  nearly  a  year  and  are  giving  us  entire  satisfaction." 


'E.  G.  Childs,  Agent,  Boston  Duck  Company.     Letter  of  November  16,  1897,  to  B.  F.  Stur- 
tevant  Company. 


364  MECHANICAL   DRAFT. 

Hotel  Iroquois,  Buffalo,  N.  Y.  —  The  direct  economic  results  of  the  introduc- 
tion of  mechanical  draft  under  the  control  of  a  proper  system  of  regulation  are 
very  forcibly  presented  in  this  plant.  The  method  of  introducing  the  air  to  the 
ashpit,  without  impingement  upon  the  grates,  is  indicated  in  Fig.  101. 


FIG.  101.    ARRANGEMENT  OF  ASHPIT  AND  DAMPER  FOR  FORCED  DRAFT  AT  HOTEL 
IROQUOIS,  BUFFALO,  N.  Y. 

'"The  boiler  plant  at  the  Hotel  Iroquois  consists  of  four  Babcock  &:  Wilcox 
water-tube  boilers,  82  nominal  horse-power  each,  with  grates  72  inches  long 
and  46  inches  wide,  equal  to  about  23  square  feet,  the  free  air  space  through 
the  gates  being  25  per  cent  of  the  total  area.  The  fan  is  a  6o-inch  Sturtevant, 
driven  by  an  independent  engine  whose  speed  is  controlled  by  the  Beckman 
system  of  automatic  valves,  holding  the  steam  pressure  within  two  or  three 
pounds  at  all  times.  The  Beckman  system  of  automatic  valves  consists  of  a 
pressure-reducing  valve,  placed  in  series  with  a  regulating  valve,  to  limit  the 
high  speed  of  the  fan,  and  to  set  the  draft  pressure  ;  and  a  by-pass  running 
around  the  regulating  valve  to  admit  steam  to  the  fan  engine,  when  the  regu- 
lating valve  is  closed,  thus  setting  the  low  speed  to  keep  up  combustion  and 
the  grates  cool.  The  maximum  speed  of  the  fan  is  655  revolutions  per  minute 
under  which  an  air  pressure  of  about  i^  inches  of  water  is  maintained  within 
the  ashpits. 


Wooley&  Gerrans,  Props,  of  Hotel  Iroquois.   Letter  of  Nov.  17,  1897,  to  B.  F.  Sturtevant  Cc 


MECHANICAL    DRAFT. 


365 


"  Previous  to  the  installation  of  this  system  of  mechanical  draft  and  regula- 
tion, four  boilers  were  needed  to  develop  the  necessary  amount  of  steam,  but 
after  its  introduction,  three  boilers  proved  to  be  all  that  were  required  to 
accomplish  the  same  results.  This  reduction  in  the  required  size  of  the  boiler 
plant  has  been  accomplished  by  a  most  remarkable  decrease  in  the  amount 
expended  for  fuel,  principally  due  to  the  ability  to  burn  a  much  larger  propor- 
tion of  hard  coal  screenings.  The  exact  record  for  two  succeeding  years  is  as 
follows  [see  Table  Nos.  139  and  140,  in  the  last  columns  of  which  are  given 
the  relative  weights  and  costs  which  in  this  case  readily  serve  as  a  basis  of  com- 
parison], the  average  load  during  the  second  year  being  30  horse-power 
additional/' 

Table  No.  139.  —  Results  of  Operation  of  Boiler  Plant  at  Hotel  Iroquois,  Buffalo,  N.Y., 
without  Mechanical  Draft. 


|               ! 

Time.                    Kind  of  Coal,      j  Number  of  Tons. 

Cost  per  Ton. 

Total  Cost  of 
Each  Kind  of  Coal 

Weight  and  Total 
Cost  of  Coal 
for  Year. 

Dec.  i,  1892 

Hard  coal   \ 
screenings,  j 
Hard  coal  \ 
screenings.  [ 

232 
601.9 

$1.25 
I.30 

;  j-  $1,072.45 
J 

} 

j*  4,7  5  1.  24  tons. 

to            < 
Nov.  30,  1893 

Soft  nut. 
Soft  nut. 
Soft  nut. 

696.95 
15.04 
1,759.6 

2.20 
2.25 
2.30 

h 
[  $9,084.92 

1 
^  $10,157.38 

j 

Soft  nut. 

1.445-75 

2.40 

Table  No.  140.  —  Results  of  Operation  of  Boiler  Plant  at  Hotel  Iroquois,  Buffalo,  N.Y., 
with  Sturtevant  Fan  and  Beckman  System  of  Automatic  Valves. 


f 

Dec.  i,  1893 
to            .4 

Hard  coal  J 
screenings,  j 
Hard  coal  ) 
screenings,  j 

1,299.95 
2,610.8 

$1-30 
1.40 

L  $5.356.24 
j 

1 

^    5,013  tons. 

Hard  nut. 

3.02 

3-5° 

} 

^    $7,680.93 

Nov.  30,  1894 

Soft  nut. 

843-03 

2.10 

t  12,333.69 

J 

I 

Soft  nut. 

255.9 

2.20 

J 

"These  results  show  an  annual  fuel  saving  of  $2,467.35,  as  the  result  of  the 
introduction  of  mechanical  draft  produced  by  a  Sturtevant  steam  fan,  controlled 
by  the  Beckman  system  of  automatic  valves,  a  showing  which  we  believe  to  be 
amply  sufficient  evidence  of  its  efficiency." 


366 


MECHANICAL    DRAFT. 


C.  Kiener  Fils,  Eloyes,  France.  —  An  exceedingly  convenient  arrangement  of 
induced-draft  in  connection  with  the  boiler  plant  of  a  large  cotton  mill  is  shown 
in  Fig.  102.  The  plant  consists  of  seven  boilers  of  special  "elephant"  type, 
each  provided' with  a  damper  in  the  flue  connection,  and  supplementary  dampers 
so  arranged  that  the  gases  may  pass  direct  to  the  chimney  or  through  the  fan, 
or  may  be  first  conducted  through  the  econo-  ^^  mizer  and  thence 
direct  to  the  chimney  or  through  the  fan  as  may  /JJj^F^^^,  be  desired.  The 
Sturtevant  fan  which  is  a  No.  100  is  driven  by  a  (f  v[\  direct-connected 

electric  motor. 


Frr,.  102.  ARRANGEMENT  OF   STURTEVANT  FAN  FOR   INDUCED  DRAFT  AT  THE   WORKS  OF 
C.  KIENER  FILS,  ELOYES,  FRANCE. 

'"Incognito"  Plantation,  La. — The  arrangement  of  a  Sturtevant  blower  for 
burning  bagasse  is  illustrated  in  the  accompanying  Fig.  103.  *  "The  boilers 
consist  of  three  batteries  of  6o-inchx22  feet  return  tubular  boilers,  each  having 
20  six-inch  tubes.  .  .  .  The  grate  surface  is  made  up  of  specially  designed 
bars,  each  of  which  is  provided  with  an  individual  blast  from  a  main  fed  by  the 


[  J.  H.  Murphy,  New  Orleans,  La.     Letter  of  November  17,  1897,  to  B.  F.  Sturtevant  Co. 


MECHANICAL    DRAFT. 


367 


pressure  blower  at  the  left  of  the  boilers.  The  requisite  amount  of  air  for  the 
total  number  of  furnaces  is  furnished  by  the  blower,  and  a  blast  gate  regulates 
the  feed  to  each  battery.  The  mill  bagasse  carrier  feeds  the  material  on  a 
sheet-iron  drag,  thence  on  to  a  drag  chain  provided  with  flights,  which  drags  the 
bagasse  along  and  feeds  it  into  hoppers  which  are  arranged  to  drop  the  contents 
automatically  into  the  furnaces." 


FIG.  103.    ARRANGEMENT  OF  BAGASSE  BURNERS  WITH  STURTEVANT  FAN,  AT  "  INCOG- 
NITO "  PLANTATION,  LA. 

Guaranty  Building  Company,  Buffalo,  N.  Y.  —  Regarding  the  operation  of 
this  plant  by  means  of  a  Sturtevant  fan,  it  is  stated  that : 

1  "  (i)  Less  air  is  required  to  produce  perfect  combustion.  For  instance,  in 
burning  one  pound  of  coal  with  chimney  draft,  we  require  from  24  to  26  pounds 
of  air,  whereas  with  mechanical  draft  we  require  only  from  18  to  20  pounds  of 
air  per  pound  of  coal.  This  represents  a  saving  of  from  10  to  15  per  cent  in 
fuel,  as  we  have  that  many  heat  units  saved  by  not  being  obliged  to  heat  this 
extra  quantity  of  air.  (2)  A  cheaper  grade  of  fuel  can  be  burned,  and  a  finer 
fuel,  because  the  fan  is  capable  of  delivering  sufficient  air  to  produce  a  perfect 
combustion.  (3)  With  a  mechanical  draft  we  can  hold  back  the  gases  which 
are  lost  with  chimney  draft,  by  closing  the  stack  damper,  and  surround  every 

'Guaranty  Building  Company,  Buffalo,  N.  V.     Letter  of  Dec.  3,  1897,  to  B.  F.  Sturtevant  Co. 


368 


MECHANICAL    DRAFT. 


MECHANICAL    DRAFT.  369 

square  inch  of  heating  surface  with  a  very  high  heat.  This  is  especially  the 
case  when  the  boilers  are  working  to  their  full  capacity,  or  close  to  it.  (4) 
Chimney  draft  requires  from  10  to  12  square  feet  of  heating  surface  to  pro- 
duce i  horse-power,  while  the  same  result  has  been  produced  by  the  mechan- 
ical draft  with  from  6  to  8  square  feet.  (5)  A  mechanical  draft  is  entirely 
under  the  control  of  the  engineer,  while  the  chimney  draft  is  not.  The  engineer 
can  thus  overcome  with  the  mechanical  draft  all  variations  in  the  outside  tem- 
perature. (6)  A  mechanical  and  automatic  draft  system  is  especially  valuable 
because  it  saves  all  the  unnecessary  expansion  and  contraction  of  the  boiler 
plates,  tubes,  etc.,  thus  prolonging  the  life  of  the  boiler.  (7)  We  have  no 
trouble  in  carrying  our  steam  at  the  required  point  needed  for  our  large  electric 
generators,  which  was  a  difficult  thing  to  do  without  mechanical  draft.  (8)  We 
are  also  able  to  avoid  all  trouble  with  smoke,  although  using  a  cheap  grade  of 
fuel.  Our  combustion  is  perfect.  Our  plant  is  a  most  economical  one." 

Stamford  Gas  and  Electric  Company,  Stamford,  Conn.  —  Two  special  Sturte- 
vant  steel-plate  steam  fans,  with  full  housings,  placed  side  by  side  with  an  inlet 
chamber  between,  comprise  this  induced-draft  plant.  Each  fan  is  driven  by  a 
direct-connected  double  upright  enclosed  engine,  with  the  wheel  upon  the  end  of 
the  extended  shaft,  the  engine  bearings  being  water-cooled.  The  entire  appa- 
ratus, as  shown  in  Fig.  104,  is  placed  on  the  level  of  the  top  of  the  boilers  and 
nearly  above  the  economizer.  The  gases,  led  from  the  back  of  the  boilers,  pass 
through  or  by-pass  the  economizer,  as  may  be  determined  by  the  position  of 
a  damper,  and  thence  enter  the  chamber  between  the  fans.  By  means  of 
dampers  therein  the  gases  may  be  caused  to  enter  the  inlet  of  either  fan,  and 
thence  pass  to  either  of  the  outlet  connections  which  unite  in  the  stack  above. 
At  this  junction  is  another  damper.  By  proper  manipulation  of  dampers  either 
fan  may  be  entirely  shut  off  and  rendered  accessible  through  doors  which  are 
provided.  The  speed  of  the  engines  is  controlled  by  special  regulating  valves 
so  as  to  always  produce  the  requisite  draft. 

The  boiler  plant  at  present  consists  of  four  2oo-horse-power  Wood  water- 
tube  boilers  set  in  two  batteries.  No  direct  tests  have  been  made,  but  the 
owners  '"have  been  experimenting  with  various  mixtures  of  coal,  and  have 
found  no  trouble  in  getting  all  the  draft  we  have  needed  so  far,  and  believe  the 
plant  you  erected  for  us  will  do  what  you  claimed  for  it."  The  fans  were 
designed  so  that  either  would  handle  the  gases  at  300°,  after  passing  through 
the  economizer,  and  maintain  a  vacuum  sufficient  to  burn  25  pounds  of  coal 
per  square  foot  of  grate. 


Stamford  Gas  and  Electric  Co. .Stamford,  Conn.  Letter  of  Oct.  27,1897,10  B.F.SturtevantCo. 


370 


MECHANICAL    DRAFT. 


Socie*te  Alsacienne  de  Constructions  mecaniques,  Belfort,  France.  —  The  ac- 
companying Fig.  105  serves  to  indicate  the  general  arrangement  of  this  plant. 
It  consists  of  six  boilers  of  the  ';  elephant "  type,  of  which  only  two  are  here 
shown.  The  fan  is  a  No.  100  Sturtevant,  driven  by  a 
belted  electric  motor  at  about  550  revolutions  per  minute. 
The  working  pressure  is  6  kilogrammes  per  square  centi- 
metre, equivalent  to  about  90  pounds  per  square  inch. 


FIG.  105.     ARRANGEMENT  OF  STURTEVANT  FAN  FOR  INDUCED  DRAFT  AT  SOCIETE 
ALSACIENNE  DE  CONSTRUCTIONS  MECANIQUES,  BELFORT,  FRANCE. 

The  general  conditions  of  operation  of  the  plant  are  as  follows  :  — 
Heating  surface  per  boiler  .         .         .         .65  square  metres. 

Maximum  production  of  steam  required  per  hour,  7,000  kilos. 
Maximum  combustion  of  coal  per  hour  .  .  1,000  kilos. 
Total  grate  surface  of  the  six  boilers  .  .  ,  14.4  square  metres. 

Each  boiler  connects  with  the  general  flue  by  a  duct  450  millimetres  by  750 
millimetres.  The  general  flue  has,  at  its  entrance  to  the  economizer  and  at  the 
chimney,  an  area  of  1.44  square  metres.  The  arrangement  of  the  dampers  is 
such  that  the  gases  may  be  caused  to  pass  by  or  through  the  economizer,  and 
thence  either  through  the  fan  or  direct  to  the  chimney. 


MECHANICAL   DRAFT.  37i 

Leon  Godchaux,  Elm  Hall  Plantation,  La.  — The  accompanying  illustration, 
Fig.  1 06,  represents  a  plant  of  bagasse  burners  installed  by  Haubtman  &  Loeb. 
It  consists  of  six  cylindrical  tubular  boilers,  each  72  inches  diameter  by  24  feet 
long.  The  necessary  air  supply  for  combustion  is  furnished  by  a  No.  9  "  Mono- 
gram" Sturtevant  blower,  driven  by  an  independent  engine,  and  arranged  as 
shown.  Each  of  the  boilers  is  provided  with  a  separate  burner  of  the  type 
known  as  a  "  Dutch  Oven."  The  entire  burner  is  surrounded  by  an  air  space 


FIG.  106.     ARRANGEMENT  OF  HAUBTMAN  &  LOEB'S  BAGASSE  BURNERS  WITH  STURTE- 
VANT FAN  AT  ELM  HALL  PLANTATION,  LA. 

in  the  brickwork,  with  which  space  it  is  in  communication  through  numerous 
small  tuyere  holes.  The  air  forced  into  the  surrounding  space,  at  a  pressure  of 
about  i  y2  ounces  per  square  inch,  is  thereby  admitted  both  above  and  below 
the  fire  in  the  required  proportion.  Hollow  blast  bars  beneath  the  grate  bars 
are  also  employed,  so  as  to  thoroughly  distribute  the  air.  The  bagasse  is  fed  to 
the  furnaces  through  hoppers,  as  shown.  Each  hopper  valve  is  provided  with  a 
counter  balance,  so  that  the  bagasse  may  be  fed  with  the  minimum  passage  of 
air  through  the  opening  during  the  process. 


372 


MECHANICAL    DRAFT. 


Otto  Colliery,  Philadelphia  &  Reading  Coal  and  Iron  Company,  Branchdale, 
Pa.  —  A  general  view  of  this  colliery  is  presented  in  Fig.  107.  The  boiler  plant, 
to  which  reference  is  here  made,  is  located  just  over  the  crest  of  the  ridge,  the 
tops  of  the  three  stacks  being  visible.  The  following  description  and  report 
of  very  carefully  conducted  tests  serve  to  make  clear  the  somewhat  novel  ar- 
rangement which  has  been  here  adopted  for  economizing  fuel  even  where  it  is 
abundant.  They  most  emphatically  indicate  the  necessity  of  mechanical  draft 
for  securing  the  desired  results  :  — 


FIG.  107.    OTTO  COLLIERY,  PHILADELPHIA  &  READING  COAL  AND  IRON  COMPANY, 
BRANCHDALE,  PA. 

1  "  Anthracite  coal  is  burned  under  all  of  the  boilers,  this  coal  being  refuse 
termed  '  rice  mixture,'  which  is  the  refuse  left  after  pea  and  buckwheat  are 
screened  out  of  it.  It  is  the  poorest  kind  of  fuel  imaginable,  and  can  only  be 
burned  by  using  blowers.  From  the  accompanying  report  of  test  the  amount  of 
ash  in  this  coal  runs  up  to  35  per  cent,  and  this  'rice  mixture'  also  contains  at 
times  10  per  cent  of  water.  The  drawing  shows  the  air  duct  but  not  the 


Thayer  &  Company,  Boston,  Mass.     Letter  of  Nov.  26,  1897,  to  B.  F.  Sturtevant  Co. 


MECHANICAL    DRAFT. 


373 


blowers,  which  are  located  at  the  end  of  the  boiler  house  in  a  separate  building. 
At  the  present  writing,  the  three  Cahall  boilers  of  250  horse-power  each  are 
set  in  their  respective  positions,  as  shown  on  the  drawing,  the  first  installation 
proving  so  satisfactory  as  to  call  for  duplicates." 

The  general  arrangement  of  these  boilers  is  indicated 
in  Fig.  1 08,  the  Cahall  boilers  being  shown  in  the  rear. 


FIG 


ARRANGEMENT  OF  CYLINDER  AND  CAHALL  BOILERS  AT  OTTO  COLLIERY,  PHIL- 
ADELPHIA &  READING  COAL  AND  IRON  COMPANY,  BRANCHDALE,  PA. 


"  In  this  plant  there  are  30  plain  cylinder  boilers,  and  the  Cahall  boilers  are 
located  between  them  and  the  chimney,  so  as  to  use  the  unabsorbed  heat  before 
it  escapes  up  the  chimney,  the  gases  being  forced  through  the  Cahall  boilers  by 
means  of  the  blower  system,  which  is  absolutely  necessary  to  the  accomplish- 
ment of  the  desired  results.  There  is  also  an  auxiliary  arrangement  for  firing 
the  Cahall  boilers  by  coal  in  the  regular  furnace  in  case  of  derangement  or 
changes  in  the  other  part  of  the  plant,  so  that  this  same  refuse  fuel  may  be 
burned  on  the  grate  in  the  regular  Cahall  extension  furnace  by  the  blower  sys- 
tem. The  air  blast  is  carried  to  the  ashpit  through  the  bridge  wall  of  each  of 
the  boilers,  care  being  taken  that  there  shall  be  a  large  mouthpiece  into  the  ash- 
pit from  the  air  duct,  so  as  to  prevent  any  contracted  blowpipe  action.  The 
blower  system  is  used  throughout  all  of  their  collieries.  Natural  draft  could  not 


374 


MECHANICAL    DRAFT. 


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MECHANICAL    DRAFT. 


375 


have  allowed  this  method  of  utilizing  waste  heat.  By  the  use  of  this  blower 
system  they  secured  an  increase  of  steam  power  capacity  amounting  to  85  per 
cent  without  any  increase  in  the  amount  of  fuel  burned.  The  gain  in  fuel 
economy  by  the  use  of  the  Cahall  boiler  with  this  system  was  74  per  cent,  that 
is  taking  that  much  out  of  the  gases  before  they  escaped.  The  use  of  this 
blower  arrangement  not  only  made  this  saving  and  this  immense  increase  in  the 
capacity,  but  it  also  showed  this  without  additional  labor,  which  in  this  single 
plant  as  shown  on  this  drawing  of  750  horse-power  of  Cahall  boilers,  showed  a 
saving  of  labor  and  mule  feed  of  $3,822  annually.  The  mule  feed  referred  to 
consists  in  the  expense  of  handling  coal.  This  plant  runs  24  hours  per  day." 

The  arrangement  of  dampers  for  admission  of  air  to  the 
ashpits  when  required,  is  shown  in  Fig.  109,  the  damper  being 
operated  from  the  front  by  means  of  a  lever.  When  the  cylinder 
boilers  are  discharging  the  gases  into  the  Cahall  boiler  they 
are  admitted  through  the  special  opening  here  shown  in  the 
side  of  the  combustion  chamber,  whence  they  pass  upward 
across  the  water  tubes  in  the  usual  manner.  The  boiler,  be- 
cause of  its  vertical  form,  becomes  in  effect  a  chimney. 

The  general  specifications  of  the  plant,  the  conditions 
existing  and  the  results  obtained  by  careful  trials  are 
.clearly  shown  in  the  following  extracts  from  the  official 
report.  The  general  results  are  presented  collectively 
in  Table  No.  141. 

108  tubes  4  inches  by  18  feet ;  mud  drum  68  inches  diameter 
by  48  inches;  top  drum  80x80  inches;  smoke  flue  in  top  drum  34  inches 
diameter  by  80  inches. 

"  Heating  surface  exposed  to  water,  2,536  square  feet.  Steam  heating  surface, 
50  square  feet. 

"  Cylinder  Boilers :  These  are  arranged  four  in  a  battery  and  two  to  a 
furnace.  The  shells  are  34  inches  by  30  feet,  set  to  expose  123.5  square  feet  of 
water-heating  surface  per  boiler.  The  grates  are  5  feet  long  and  provide  15.8 
square  feet  to  each  boiler. 

"  Stacks :  Each  battery  of  four  boilers  is  provided  with  a  stack  34  inches 
diameter  by  36,  51  and  55  feet  high,  respectively.  These  stacks  were  not  in 
use  on  the  tests  hereafter  described. 

"The  Cahall  boiler  has  a  stack  38  inches  in  diameter  which  was  53  feet  6 


FIG.  109. 
Cahall  Boiler 


i  Report  of  Mr.  Jay   M.  Whitham,  on  Trials    of   Cahall   Boiler    using    Waste    Heat   from 
Cylinder  Boiler,  1896. 


376 


MECHANICAL    DRAFT. 


inches  high,  above  the  grates,  on  trials  438,  439  and  440,  and  78  feet  6  inches 
high  on  trials  457,  458  and  459. 

"Fan  Blast:  The  draft  was  supplied  by  a  fan  blast  under  the  grate,  which, 
on  the  trials  noted,  supplied  air  for  the  30  cylinder  boilers  at  this  colliery. 

"  Results  of  the  Trials :  Trial  438,  shown  on  annexed  trial  sheet,  was  made 
on  8  cylinder  boilers. 

"Trial  458  was  made  on  12  cylinder  boilers.  On  these  trials  the  chief 
items  of  interest  are:  Temperature  of  waste  gases  about  1,600°  Fahr.  Horse- 
power developed  per  cylinder  boiler,  33  to  35.  Evaporation,  212°  Fahr.,  per 
pound  dry  coal,  3.77  pounds. 

"Trials  439  and  457  relate  to  the  horse-power  developed  by  the  Cahall  boiler 
when  receiving  waste  gases  from  the  cylinder  boilers.  The  trials  of  the  cylinder 
boilers  and  the  Cahall  boilers  were  made  simultaneously. 

"  Trial  439  shows  the  duty  of  the  Cahall  boiler  when  coupled  to  8  cylinder 
boilers,  and  trial  457  when  coupled  to  12  boilers. 

"  The  chief  items  of  interest  are  as  follows  :  — 

No.  of  cylinder  boilers  supplying  waste  heat 
Temperature  of  gases  entering  Cahall  setting 
Temperature  of  Cahall  stack         .... 
Horse-power  developed  by  cylinder  boilers    . 
Horse-power  developed  by  Cahall  boiler 
Total  horse-power  developed 

Gain  in  capacity  by  use  of  Cahall  boiler,  without 
change  in  labor  employed  or  fuel  burned 

"The  above  results  are  shown  more  fully  in  trials  440  and  459,  which  repre- 
sent the  combination  of  cylinder  and  Cahall  boilers.  The  boiler  horse-power 
developed  from  hour  to  hour  was  as  follows  for  the  Cahall  boiler,  when  supplied 
with  gases  from  8  cylinder  boilers,  viz.  :  — 


No.  439. 

No.  457- 

8 

12 

i,  600°  Fahr. 

i,  6  1  2°  Fahr. 

732°  Fahr. 

711°  Fahr. 

280.4 

393-1 

207.6 

334.1 

488.0 

727.2 

74  per  cent. 


&5  per  cent. 


I  St. 
2d. 

3d. 

4th. 

5th- 

6th. 

7th. 

8th. 


Cahall  boiler  made 


Average  for  8  hours, 


267.0  horse-power,  fires  clean. 

255.0 

141.8  "          "         cleaning. 

257.0  "         "        fires  clean. 

224.8  "          " 
175.0  " 
183.3  " 

156.9  "         "        cleaning. 
207.6  horse-power. 


MECHANICAL   DRAFT.  377 

"When  supplied  with  waste  heat  from  12  cylinder  boilers,  the  Cahall  boiler 
developed  power  as  follows :  — 

Hour. 

ist.       Cahall  boiler  made          .          367.4  horse-power. 

2d.  .          300.4       "         "      cleaning. 

3d.  .          384.4       " 

4th.  .          317-3        "         "      cleaning. 

5th.  "         "         "  .          351.1 

6th.  "         "         "  .  •        283.9       "         "      cleaning. 

7th.  .          334.0 

8th.  "         "         "  .          335.0       " 

Average  for  8  hours,          334-i   horse-power. 

"  Summary  :  i.  The  cylinder  boilers  are  run  to  develop  from  33  to  35  horse- 
power. 

"  2.  The  cylinder  boilers  by  themselves  evaporate  about  3.77  pounds  of  water 
per  pound  of  dry  coal. 

"3.  The  combination  of  cylinder  boilers  and  Cahall  boilers,  the  latter  using 
waste  heat  only,  permits  an  evaporation  of  6.98  pounds  of  water  per  pound  of 
dry  coal. 

"  4.  The  waste  gases  enter  the  Cahall  setting  about  1,600°  Fahr.,  and  leave  it 
about  700°. 

"5.  The  use  of  waste  gases  by  the  Cahall  boiler  increases  the  available 
horse-power  of  the  plant  from  74  to  85  per  cent,  according  to  the  number  of 
boilers  used  for  supplying  the  waste  heat. 

"6.  The  250  horse-power  Cahall  boiler  using  waste  gases  from  8  cylinder 
boilers  developed  207.6  boiler  horse-power,  and  when  supplied  by  12  boilers 
it  developed  334.1  horse-power,  or  33.6  per  cent  above  its  rating. 

"7.  The  fuel  used,  called  a  "rice  mixture,"  consisted  of  20  per  cent  slate 
pickings,  8  per  cent  buckwheat,  46  per  cent  rice  coal  and  26  per  cent  dirt.  It 
contains,  as  used  at  this  colliery,  from  6.25  to  9.5  per  cent  moisture,  and  from 
32.4  to  34  per  cent  ash  and  refuse. 

"  It  is  burned  with  a  strong  fan  blast." 


INDEX. 


ABSOLUTE  ZERO,  7. 
Abstractors,  116. 
Air,  admission  above  fire,  78. 
composition,  16. 
efflux,  144. 

coefficient,  151. 

coefficient  of  contraction,  151. 

influence  of  form  of  orifice,  151. 

work  required,  152. 

excess,  loss  on  account  of,  So. 

expansion  by  heat,  9. 

for  dilution,  25. 

friction  of,  1 55. 
heaters,  116. 

Ellis  &  Eaves,  293. 
leakage,  177. 

movement  in  long  pipes,  1 55. 
pre-heating,  80,  93. 
pressure,  141. 

and  horse-power  required  to  compensate 
for  friction  in  pipes,  1 56-8  (table). 

relation     to    indicated    horse-power    per 

square  foot  of  grate,  171. 
properties,  32. 

saturated  mixtures  of,  and  vapor,  9. 
supply  for  combustion,  225. 

admission  above  fire,  78. 

calculation,  24,  27. 

calculation  from  analysis,  28. 

dilution,  influence  on  temperature,  34. 

effect  of  increased,  upon  temperature  of 
gases,  83. 

excess,  loss  on  account  of,  80. 

ideal  temperature  with    different  degrees 
of  dilution,  34. 

influence  on  efficiency,  72,  84,  134. 

insufficiency,  influence  of,  27. 

reduction  by  mechanical  draft,  83. 

tests  by  Donkin  &  Kennedy,  26. 

tests  by  Whitham,  27. 

with  different  rates  of  combustion,  137. 

with  mechanical  draft,  27. 
temperature,    influence   on    movement,    153 

(table), 
velocity  of  efflux,  141. 

under    given    pressure     in    inches,    150. 


Air,  velocity  of  efflux,  under  given  pressure  in 

ounces,  148. 

volume  and  horse-power  under  given  press- 
ure in  ounces,  148  (table), 
volume,  9. 
weight,  9. 

work  required  to  move,  152. 
Alsacienne,  Societe,  de  Constructions  mecani- 

ques,  mechanical  draft  plant,  370. 
American   Line    Pier  14,   induced-draft  plant 

and  test,  293. 
American  stoker,  333. 
Anemometer,  Wollaston,  163. 
Anthracite  coal,  see  Coal,  anthracite. 
Application   of   the   Sturtevant  fans   for   me- 
chanical draft,  273. 
Artificial  fuels,  54. 
Ash,  carbon  in,  66. 
influence  of,  66. 
Ashpit  dampers,  275. 
Atomic  theory,  17. 

BAGASSE,  39. 

burners,  Cook's,  310. 

Gordon  hollow-blast  grate,  301. 
Haubtman  &  Loeb's,  371. 
Murphy's,  366. 

calorific  value,  40-2. 

composition,  39. 

diffusion,  39. 

fuel  value,  42. 

mill,  39. 

moisture  in,  40,  41. 

Berlin,  S.  S.,  mechanical  draft  plant  tests,  169. 
Bituminous  coal,  see  Coal,  bituminous. 
Blast  grates,  Gadey,  302. 

Gordon,  199. 
Blowers,  see  also  Fans. 

definition,  199. 

Boiler  settings,  leakage  of  air  through,  177. 
Boilers,  steam. 

combustion  rate,  influence  of,  in. 

convection  of  furnace  heat,  102. 

draft,  conditions  of,  165. 

mechanical,  influence  of,  123. 

economy  of  high  rates  of  combustion,  132. 


38° 


INDEX. 


Boilers,  steam,  efficiency,  95,  129. 
measure  of,  95. 
relative,  96. 
sources  of,  107. 
ultimate,  123. 

of  different  types,  97. 

evaporation,  rate,  with  mechanical  draft,  101. 
evaporative  rates  per  square  foot  of  heating 

surface,  131. 
performance,  111. 

grate  area  and  heating  surface,  109,  131. 
heat  balance,  97. 
heat,  disposition  of,  106. 

distribution  of,  104. 
heating  surface,  and  grate  area,  109,  131. 

per  horse-power,  101. 
horse-power,  98. 
leakage  of  air,  177. 
powdered  fuel  furnaces,  123. 
radiation  of  furnace  heat,  102. 
rating,  98. 
retard ers,  1 18. 
Serve  tubes,  1 20. 
stokers,  mechanical,  122. 
surface  ratios,  109. 

and  combustion  rate,  138. 
influence  of,  in. 
temperatures  in,  105,  117. 
tube  heating  efficiency,  118. 
tubes,  ribbed  vs.  plain,  120. 
Boston  Duck  Co.,  mechanical  draft  plant  and 

tests,  361. 
Boston    Woven  Hose    Co.,    mechanical    draft 

plant  and  test,  356. 
Bricquettes,  58. 
British  thermal  unit,  4. 

Brown,    John    &    Co.,   Ltd.,   mechanical    draft 
plant  and  test,  317. 

CANNEL  COAL,  see  Coal,  cannel. 
Carbon,  16. 

heat  of  combustion,  30. 

in  ash,  66. 

properties,  32. 

union  with  oxygen,  19. 
Carbonic  aci'd,  composition,  19. 

properties,  32. 
Carbonic  oxide,  composition,  19. 

heat  of  combustion,  30. 

loss  of  efficiency  on  account  of,  75. 

produced  by  excessive  firing,  76. 

properties,  32. 

"  Centennial  "  standard  horse-power,  99. 
Central    Unidad,  bagasse  burner,   mechanical 

draft,  310. 
Charcoal,  55. 

Charleston,  U.  S.  S.,  tests  of  mechanical  draft 
fans,  218. 


Cheney  Brothers,  induced-draft  plant  and  test, 

114,302. 
Chimney,  advantage  of  omission,  233. 

cost,  relatively  to  mechanical  draft,  124,  237, 

3°5- 

design,  181. 

draft,  see  Draft,  chimney, 
efficiency  vs.  fan,  191. 

height  to  produce  certain  rate  of  combus- 
tion, 189. 
Chimneys,  see  also  Draft  chimney. 

capacity  in  horse-power,  185  (table). 
Cleveland  Iron  Mining  Co.,  mechanical  draft 

plant  and  test,  314. 
Closed  ashpit  system,  214. 
Closed  fire  room  system,  217. 
Coal,  see  also  P'uels. 
anthracite,  48. 

composition,  21,  48. 

relative  rates  of  combustion  of  small,  171. 

small,  factor  in  burning,  69. 
bituminous,  46. 

caking,  47. 

cannel,  47. 

composition,  21,  46. 

non-caking,  47. 

buckwheat,  tests  with  mechanical  draft,  71. 
cannel,  composition,  47. 
calorific  value,  50  (table), 
classification,  chemical,  45. 

geographical,  49. 

size,  68. 

combustion,  heat  losses,  incident  to,  85,  89. 
composition,  21,  50  (table). 

proximate,  46. 

ultimate,  46. 

cost,  equivalent  for  different  rates  of  evap- 
oration, 91  (table), 
efficiency,  see  also  P'uels,  efficiency. 

commercial,  87. 

comparative,  62,  65. 
formation,  44. 
fuel  value,  50  (table), 
heat  of  combustion,  31. 
lignite,  21. 

composition,  21,  45,  46. 
losses  incident  to  combustion,  64. 
moisture  in,  67. 

pea,  tests  with  mechanical  draft,  7 1 . 
semi-anthracite,  48. 
semi-bituminous,  21,  48. 
size,  influence  on  draft,  67. 

influence  on  efficiency,  67. 
sizes,  68. 
small,  burning,  305. 

draft  for,  67. 

relative  efficiencies,  72. 
values,  relative,  90. 


INDEX. 


Coal,  small. 

relative  efficiencies,  72. 

values,  relative,  90. 
Coke,  55,  56. 
Combustion,  16-35. 

air  required  for,  22. 

data,  23. 

definition,  16. 

double,  94. 

economy  of  high  rates,  132. 

efficiency  with  mechanical  draft,  69,  72,  91, 
123,  233,  327. 

forced,  see  Draft,  mechanical. 

heat  of,  30. 

high  rates,  efficiency  with  mechanical  draft, 

'35- 

ideal  temperature,  32. 
losses,  incident  to,  64. 
rate  of,  130-140. 

for  different  boilers,  130. 
for  various  heights  of  chimney,  189. 
increased  by  mechanical  draft,  233. 
relative  to  draft,  169,  175,  177. 
relative  to  square  root  of  pressure  differ- 
ence, 173. 

with  different  surface  ratios,  138. 
on  torpedo  boats,  173. 
with  small  anthracite  coals,  171. 
Cook  bagasse  burner,  310. 
Convection  of  furnace  heat,  102. 
Coxe  mechanical  stoker,  72,  170,  305. 

travelling  grate,  72. 
Crane    &   Breed    Mfg.    Co.,    mechanical  draft 

plant,  333. 
Crematory,  garbage,  353. 

body,  358. 

Cross  Creek  Coal  Co.,  see  Deringer  Colliery. 
Crystal  Water  Co.,  mechanical  draft  plant,  277. 

DANIA,  S.  S.,  saving  by  forced  draft,  217. 
Darling,  L.  B.,  Fertilizer  Co.,  mechanical  draft 

plant,  313. 
Deringer  Colliery,  mechanical  draft  plant  and 

test,  305. 

Dilution,  air  required  for,  25. 
Disposition  of  heat  in  steam  boilers,  106. 
Draft,  141-178. 
artificial,  224. 
chimney,  179-194. 

Gale's  theory,  184. 

height  to  produce  certain  rate  of  combus- 
tion, 189  (table). 

in  chimney  100  feet  high,  180  (table). 

Kent's  formulae,  183. 

principles,  179. 

Smith's  formulae,  187. 

temperature,  influence  of,  180. 

theory,  181. 


Draft. 

conditions,  165. 
definition,  141. 

forced,  see  Draft,  mechanical, 
gauges,  161. 

induced,  see  Draft,  mechanical, 
instruments  for  measurement,  161. 
measurement,  159. 
mechanical,  195-222. 
adaptability,  228. 
advantages,  135,  223-242. 
general,  223. 
summary  of,  241. 
air  pressures  produced  by,  171. 
air  supply,  27,  135,  225. 
air  supply,  influence  on,  72. 
application  for  burning  small    anthracite 

coals,  69. 

arrangement,  typical,  1 24. 
burning  cheap  fuels,  234. 
carbonic  acid,  prevention  of,  76. 
climatic  conditions,  230. 
closed  ashpit  system,  214. 
closed  fire  room  system,  217. 
combustion,  efficiency  of,  233. 
combustion  rate  increased,  233. 
compared  with  chimney,  191. 
controllability,  229. 
cost,  economy  of,  237. 

relatively  to  chimney,  124,  237. 
decreased    size   of    boiler   required,    127, 

238. 

economizer,  with,  1 14,  236. 
economy,  due  to,  225. 

of  high  rates  of  combustion,  133. 
in  quantity  of  fuel,  235. 
efficiency,  27. 

compared  with  chimney,  193,  232. 
influence  on  ultimate,  of  boilers,  123. 
evaporation,  rate  of,  101. 
factor  in  burning  small  anthracites,  69. 
fans  applied  for,  273-278. 
fans  for,  243-272. 
flexibility,  230. 
forced,  typical  arrangement,  275. 

vs.  induced,  221. 
heat  transmission,  102. 
history,  198. 

important  in  utilization  of  cheap  fuels,  91. 
induced,  fan  applied  for,  262. 
system,  220. 
vs.  forced,  221. 

influence  on  ultimate  efficiency,  1 23. 
mechanical  stokers,  economy  of,  235. 
methods  of  application,  213. 
natural,  vs.,  226. 
necessary  for  small  coals,  69. 
necessity,  108,  228. 


382 


INDEX. 


Draft,  mechanical. 

operating  expense,  economy  in,  240. 

portability,  231. 

power  required  in  U.  S.  Navy,  291. 

pressure  chart,  285. 

recorder,  164,  285. 

reduced    temperature   of    escaping   gases, 
with,  83. 

reduction  of  boiler  plant  by,  127. 
of  space  by,  128. 

retarders,  with,  119. 

salability,  232. 

saving  by,  91,  123,  126,  217,  327. 

smoke  prevention,  92,  236. 

space,  economy  of,  237. 

stokers,  with,  121. 

ventilation,  240. 

waste  heat,  utilization  of,  236. 

with  thick  fires,  140. 
natural,  195. 

see  also  Draft,  chimney. 

vs.  mechanical,  226. 
object,  ultimate,  167. 
pressure,  141. 
recorder,  164. 

relation  to  rate  of  combustion,  175,  177. 
required  for  combustion  of  different  kinds 

of  coal,  176. 

required  for  steam  boilers,  166. 
retarders,  with,  120. 
small  coals,  with,  67. 
velocity,  141. 
Dust  destructor,  324. 

EAVES  HELICAL  DRAFT,  317. 
Economizers,  112. 

surface,  influence  of  conditions  of,  1 1 5. 

with  mechanical  draft,  236. 
Efflux  of  air,  144. 
Electric  fans,  see  Fans. 
Elementary  substances,  atomic  weights,  18. 

symbols,  18. 

Ellis  &  Eaves  system,  293,  297. 
Engines,  double  upright,  enclosed,  260. 

steam  per  horse-power,  98. 
Evaporation,  factors  of,  63  (table). 

rates  for  equivalent  cost  of  coal,  91  (table). 

unit  of,  6 1 . 
Exhausters,  see  also  Fans. 

definition,  199. 

FACTORS  OF  EVAPORATION,  63  (table). 
Fan  wheel,  see  Fans. 
Fans,  198. 

see  also  Draft,  mechanical. 

capacity  area,  205. 

centrifugal,  199. 

design,  204. 


Fans, 
disc,  199. 
efficiency  compared  with  chimney,  193,  224, 

232. 
electric,  "Monogram"  pattern,  271. 

steel-plate  pattern,  272. 

methods  of  application,  213. 

"Monogram"  blower,  245. 

on  adjustable  bed,  246. 

with  engine,  247. 
peripheral  discharge,  199. 
power  to  operate,  202. 
pressure,  created  by,  200. 
propeller,  199. 
proportions,  205. 

relation  of  volume,  weight  and  pressure  of 
air  and  speed  and  power  of  a  fan  with 
air  at  different  temperatures,  210 
(table). 

special  duplex  steel-plate  steam  fan,  263. 
three-quarter   housing    with    steel-plate 

bottom  and  horizontal  engine,  270. 
with  double-enclosed  engine,  259. 
cast-iron  steam  fan  with  double-horizontal 

engine,  265. 
steel-plate  steam  fan  with  double-enclosed 

engine,  257,  259,  261. 
with  double  open-type  engine,  264. 
with  horizontal  engine,  257. 
with  single  engine,  256. 
with  upright  compound  engine,  263. 
square  inches  of  blast,  205. 
steam  jets,  vs.,  196. 

steel-plate  blower  with  overhung  pulley,  249. 
on  adjustable  bed  with  engine,  250. 
exhauster,  251. 

inlet  connection,  with,  255. 
steam  fan,  252. 

engine  enclosed,  with,  254. 
three-quarter  housing,  267. 

double-upright  engine,  with,  269. 
steel-plate  bottom,  with,  269. 
steel  pressure  blower,  244. 
temperature,  influence  upon  operation,  209, 

210. 

theory,  200. 
types,  199. 

volume  delivered,  201. 
wheel,  249. 
work  done  by,  201. 
Farr  Alpaca  Co.,  mechanical  draft  plant  and 

test,  337. 

Feed  water  heaters,  112. 
Fire,  thickness  of,  140. 
Firing,  excessive,  76. 

frequency,  influence  of,  73. 
Flue  gas,  see  Gas,  flue. 
Forced  draft,  see  Draft,  mechanical. 


INDEX. 


383 


Fuel,  powdered,  furnaces,  123. 
quality,  effect  of,  87. 
saving  by  mechanical  draft,  72,  91,  327. 
Fuel  gas,  see  Gas,  fuel. 
Fuels,  36-58. 
see  also  Coal. 

air  required  for  combustion  of,  25. 
artificial,  36,  54,  57. 
burning,  cheap,  234. 
efficiency,  59-95,  60  (table). 

see  also  Coal,  efficiency. 

air,  admission  above  fire,  78. 

air  supply,  influence  of,  72. 

ash,  influence  of,  66. 

carbonic  oxide,  loss  on  account  of,  75. 

combustion  of,  20. 

commercial,  87. 

commercial  value  of  losses,  89. 

composition,  21. 

definition,  36. 

draft  required  for,  176. 

effect  of  quality  of  fuel,  87. 

evaporation,  rates  of,  for  equivalent  cost 
of  coal,  91. 

excess  of  air,  loss  on  account  of,  80. 

firing,  frequency  of,  73. 

influences  affecting,  85. 

loss    due     to    temperature    of     escaping 
gases,  83. 

measure  of,  59. 

moisture  in  coal,  influence  of,  67. 

relative  values  of  coals,  90. 

size  of  coal,  influence  of,  67. 

smoke,  loss  on  account  of,  74. 
natural,  36. 
patent,  57. 
Furnaces,  down-draft,  94. 

GADEY  AIR  GRATE,  301. 
Garbage  crematory,  353. 
Gas  analysis,  with  steam  jet  and  fan,  197. 

fuel,  56,  57. 

natural,  44,  53. 
Gases,  flue  analysis,  28. 

at  B.  F.  Sturtevant  Co.,  76. 
temperature,  loss  due  to,  83. 
tests  by  Donkin  &  Kennedy,  26. 

pressure,  141. 

velocity  of  efflux,  141. 
Gauge,  water,  161. 
Glens  Falls  Paper  Mill  Co.,  mechanical  draft 

plant,  331. 

Godchaux,  Leon,  bagasse  burner,  371. 
Gordon  hollow  blast  grate,  299. 
Grate,  length  effect  of  on  efficiency,  139. 

surface,  relation  to  heating  surface,  131.  . 
Grates,  small  anthracite,  for,  69. 

travelling,  test  of  Coxe,  7 1 . 


Guaranty     Building     Co.,    mechanical     draft 
plant,  367. 

HAUBTMAN  &  LOEB'S  BAGASSE  BURNKR,  371. 

Heat  Balance,  97. 

Heat,  combustion,  of,  30. 

distribution  in  steam  boilers,  104. 

expansion  of  steam,  by,  7. 
of  water,  by,  2. 

latent,  12. 

mechanical  equivalent  of,  5. 

sensible,  12. 

specific,  3. 

total,  12. 

unit  of,  4. 
Heaters,  air,  1 16. 
Heating  feed  water,  per  cent  of  saving  by,  113. 

surface  in  steam  boilers,  101. 

relation  to  grate  surface,  131. 
Height  of   water  column  in  inches  correspond- 
ing to  various  pressures  in  ounces,  160. 
Hook  draft  gauge,  162. 
Holyoke  Street  Railway  Co.,  fans,  263. 

mechanical  draft  plant  and  test,  335. 
Horse-power  of  steam  boilers,  98. 
Hot-draft  apparatus,  116. 
Hotel    Iroquois,   mechanical  draft   plant   and 

test,  364. 

Howden  system,  341. 
Hydrogen,  30,  32. 

IDEAL  TEMPERATURE  of  combustion,  32. 

dilution,  influence  of,  34. 
Induced  draft,  220. 

see  also  Draft,  mechanical. 
"  Incognito  "  Plantation,  bagasse  burner,  366. 
International    Navigation   Co.,   see  American 

Line,  St.  Louis,  St.  Paul,  Kensington. 
Iroquois,  hotel,  mechanical    draft   plant   and 

test,  364. 

JONES  UNDER-FEED    MECHANICAL    STOKER, 

3*4- 

Kaf er  system,  29  r . 

Kiener,  C.,  Fils,  mechanical  draft  plant,  366. 
Kensington,  S.  S.,  mechanical  draft  plant  and 

test,  348. 
Knickerbocker    Lime    Co.,    mechanical    draft 

plant,  354. 

LATENT  HEAT,  12. 
Leakage  of  air,  177. 
Lignite,  see  Coal,  lignite. 

MARLAND  WARM-BLAST  APPARATUS,  116. 

Marsh  gas,  30. 

Mechanical  draft,  see  Draft,  mechanical. 

Mechanical,  equivalent  of  heat,  5. 

Megass,  see  Bagasse. 

Moisture  in  coal,  67. 


384 


INDEX. 


NATURAL  GAS,  see  Gas,  natural. 

Navy,  U.  S.,  Sturtevant  fans  on  vessels  of,  216, 

287,  290,  291. 
Nitrogen,  16,  32. 

OLEFIANT  GAS,  30. 

Orifice,  influence  of  form  on  efflux,  151. 

Otto    Colliery,    mechanical    draft    plant   and 

test,  372. 
Oxygen,  16,  32. 

union  with  carbon,  19. 

PACIFIC  MILLS,  boiler  tests,  116. 
Patent  fuels,  see  Fuels,  patent. 
Peat,  42. 

Petroleum,  21,  44,  53. 

Philadelphia  &  Reading  C.  &  I.  Co.,  mechani- 
cal draft  plant  and  test,  372. 
Pilot's  tube,  165. 
Polyphemus,  H.  M.  S.,  experiments  with  forced 

and  induced  draft,  221. 

Pope  Tube  Co.,  mechanical  draft  plant,  341. 
Pressure  gauges,  161. 
head,  143. 
in  inches  of  water  corresponding  to  various 

pressures  in  ounces,  160  (table), 
in   ounces  corresponding   to  various   press- 
ures in  inches  of  water,  160  (table). 
and  velocity,  141. 
Puritan,  U.  S.  S.,  fan  for,  265. 

RADIATION  OF  FURNACE  HEAT,  102. 
Retarders,  118,  296. 

Riordon    Paper  Mills,  Ltd.,  mechanical  draft 
plant,  360. 

SEMI-BITUMINOUS  COAL,  see  Coal,  bitumin- 
ous. 

Sensible  heat,  12. 
Serve  tubes,  1 20,  293. 
Shoreditch  Dust  Destructor,  324. 
Smoke,  analysis,  74. 

loss  on  account  of,  74. 

prevention,  92,  236. 
Societe  Alsacienne  de  Constructions  mecani- 

ques,  mechanical  draft  plant,  370. 
Southwark,  S.  S.,  mechanical  draft  plant,  348. 
Specific  heat,  3. 
Stamford  Gas  and   Electric   Co.,   mechanical 

draft  plant,  369. 
Steam,  7-15. 

boilers,  see  Boilers,  steam. 

bulk,  7. 

composition,  7. 

expansion  by  heat,  7. 

fans,  see  Fans. 

flow  of,  13. 

jets,  195. 

latent  heat,  12. 


Steam. 

loss  of  pressure  in  pipes,  14. 

per  horse-power  for  steam  engines,  98. 

pressure  chart,  284,  322,  330. 

properties  of  saturated,  10  (table). 

sensible  heat,  12. 

specific  heat,  12. 

temperature,  10. 

thermal  units  in,  10. 

total  heat,  12. 

weight,  7. 

Steam-pipe  coverings,  14. 
Steel-plate  fans,  see  Fans. 
St.  Louis,  S.  S.,  mechanical  draft  plant  and 

test,  341. 
Stokers,  mechanical,  121. 

American,  333. 

Coxe,  72,  170,  305. 

Jones,  314. 

mechanical  draft,  with,  235. 

Wilkinson,  test  of,  27. 

St.  Paul,  S.  S.,  mechanical  draft  plant,  341. 
Straw,  38. 
Sturtevant,  B.  F.  Co.,  boiler  test,  278. 

carbonic  oxide  in  gases,  76. 

combustion  rate,  172. 

draft  in  boiler  plant,  172. 

mechanical  draft  plant,  278. 
Swatara,  U.  S.  S.,  air  pressures,  216. 

mechanical  draft  arrangement,  290. 

TABULAR  VIEW  of  results  of  combustion  of 

coal,  86. 
Tan,  38. 
Temperature,  coal  fire,  of,  103. 

boiler  in,  105. 

ideal,  of  combustion,  32. 

influence  on  movement  of  air,  153. 

tubes,  in,  of  marine  boiler,  105. 
Thermal  unit,  4. 
Tube-heating  efficiency,  118. 

UNION  TRACTION  Co.,  mechanical  draft  pi  n^ 

320. 
Unit  of  evaporation,  61. 

of  heat,  4. 

United    States  Cotton  Co.,  /mechanic 
plant  and  test,  91,  327. 

efficiency,  91. 

United  States  Navy,  see  Navy,  U.  S. 
VELOCITY  HEAD,  143. 

of  efflux  of  gases,  141. 

pressure,  and,  141. 
Vena  Contracta,  151. 
Ventilation  by  mechanical  draft,  240. 

WALDO,    STEAMER   L.  C.,  mechanica. 

test,  297. 
Warm-blast  apparatus,  116. 


f 


INDEX. 


385 


Washington  Garbage  Crematory,  353- 
Washington  Incinerator,  3158. 
Water,  1-6. 

bulk,  i. 

column,  height  due  to  unbalanced  press,  in 
chimney,  180. 

composition,  i,  17. 

density,  2. 

expansion  by  heat,  2. 

gauges,  161. 

head  of,  corresponding  to  pressure,  5. 

impurities  in,  6. 


Water. 

pressure  of,  5. 

specific  heat,  3. 

thermal  units  in,  4. 

weight,  i. 
Wilkinson,  automatic  mechanical  stoker,  137. 

Wollaston  anemometer,  163. 
Wood,  37. 

fibre,  conversion  into  coal,  44. 

ZERO,  ABSOLUTE,  7. 


TJ 

335> 


Sturtevant  -  Mechanical  draft. 

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